The ASCII lines in the header end with '\n'.

First line should be the magic:
NRRD0004

Rest of header will be a sequence of plain text lines, terminated by
an empty line ('\n' alone).  Some header lines give per-array
information, some give per-axis information.

Note that the distinction between scalar and non-scalar images is
given by the per-axis "kinds" field, so that is where further
information is given about the constraints associated iwth non-scalar
image data.

============================================================
The basic fields that should be there are:

----------------
type: <string>

Gives the type of the scalar values in the array.  Current values are:
signed char
unsigned char
short
unsigned short
int
unsigned int
long long int
unsigned long long int
float
double

----------------
dimension: <int>

Gives the dimension of the array.  Must be greater than 0.  Current
code has a #define that limits the value to <= 16.  The per-axis fields
will have this many pieces of information.

----------------
encoding: <string>

How scalar values are encoded on disk.  Currently, should only see 
"encoding: raw".

----------------
endian: <string>

Endian-ness of raw values saved on disk.  Possibilities are:
little
big

----------------
space dimension: <int>

Gives the dimension of the world-space in which the array or oriented
(For now we're not allowing the named spaces that are given with the
"space: " field).  The points and vectors given by "space origin"
and "space directions" will have this many coefficients, so this
field has to precede "space origin" and "space directions".

----------------
space origin: (<float1>,<float2>,...,<floatN>)

Gives the location of the center of the first sample (with index
space coordinates (0,0,0,...,0).  The number of coefficients 
separated by commas and bounded by parens (with optional whitespace
throughout) is given by the "space dimension" field.  Each coefficient
is a floating point value (of the sort produced by printf("%g", val))

============================================================
The per-axis fields that should be there are as follows.  The ordering
of these fields is always *fast* to *slow*, and the number of pieces
of information is always equal to the image dimension.

----------------
sizes: <int1> <int2> ... <intD>

The number of samples along each axis.  Each integer value must be >= 0.

----------------
kinds: <string1> <string2> ... <stringD>

The kind of information represented by the samples along each axis. The
possible values are:

2-vector
3-vector
4-vector
2D-symmetric-matrix
2D-matrix
3D-symmetric-matrix
3D-matrix
space

The "space" kind is for a typical axis of the image, with samples that
subtend some region of the spatial domain of the image.  All the other
kinds are the sequence of scalar values that compose a non-scalar
image value (vectors and tensors).

We currently allow there to be *at* *most* *one* axis with a
non-"space" kind.  We can have non-scalar image values, but we don't
have cartesian products of non-scalar image values.  The non-"space"
kind can be anywhere in the axis ordering.  If all the axis kinds are
"space", then then image data is for scalars, else it is for
non-scalar data.

In scalar data, the image "dimension" must equal the "space dimension". 
In non-scalar data, the image "dimension" must equal *one* plus 
the "space dimension".  That is, the spatial axes have to form a
basis, not necessarily orthonormal, for world space (the vectors
are defined by the "space directions" field, below).  Thus, a
3-D scalar image can't reside in a 2-D or 4-D world space, nor can
a 3-D vector image reside in a 3-D or 5-D world space.

We are currently not allowing the "measurement frame" field that
identifies the coordinate system in which these are measured relative
to the image orientation, so nothing should be assumed about the space
in which these coefficients are measured.  The order of coefficients
for these non-scalar kinds is:

2D-symmetric-matrix: Mxx Mxy Myy
2D-matrix: Mxx Mxy Myx Myy
3D-symmetric-matrix: Mxx Mxy Mxz Myy Myz Mzz
3D-matrix: Mxx Mxy Mxz Myx Myy Myz Mzx Mzy Mzz

----------------
space directions: (<x1>,<x2>,...,<xN>) (<x1>,<x2>,...,<xN>) ...
(and at most one of these vectors is actually "none")

These give the "direction" in world-space between successive samples
along each axis: the columns of the index-to-world transform matrix.
There is one per image axis, even for the axis of coefficients of
non-scalar image values.  Because such an axis really has no spatial
extent, for this and only this axis, the space direction vector is
instead the string "none".

============================================================
Field ordering:

The fields should appear in this order:
type
dimension
space dimension
sizes
space directions
kinds
endian
encoding
space origin