Add unit conversion tables for model inputs and outputs to 'defs'

[jrawbits]

Refer to Issues #122 and #124. Add standard table files to the 'defs' for each VIsionEval model so data in the input files can be automatically converted to the required internal units from units that are more convenient for the user to provide (obviating manual conversion of units as a data preparation step).

[jrawbits]

This is implemented in the "extract" function in VEModel. It needs to be generalized to query results as well.

[jrawbits]

I'm going to close this issue and refer people to #18 which addresses unit conversion in query results specifically.