Indexing support needs to be written

[billyc]

Transcad-style indexing is not yet supported. Design details:

• Where & how should indices be stored? Attributes vs. tables.
• What's the API for accessing data via an index?

[billyc]

Release 0.1 includes a new Mapping API which covers this. Mappings are stored in the file, and can be used for any matrix stored within that file.

See https://sites.google.com/site/openmodeldata/omx-file-specification/apis