This to do for the Vis 2012 submission

3rd order tensors: 2x2x2 and 3x3x3 (done)

dynamic-length sequences (done, except for loading and smart memory management)

curl 2D and 3D

support iteration over image indices

loading sequences

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Plan for loading sequences

check proxy files when converting from AST to Simple representation; use default representations
when no proxy is specified.

get rid of lift/eval phases (although we might want to address teh polyvariance of fields
at that stage)

Need to update nrrd info to allow for default representations where various properties
are not specified (origin and sizes).

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Currently, unused inputs are deleted, which may be surprising to the user.  Instead, we should
just generate a compile-time warning about unused inputs.