Add spreadsheet functionality to save data

Please add the functionality that is in the spreadsheet to save data, especially for .csv data. I know of two holes:

• Add the ability to only write out selected cells or points.
• Add the ability to only write out subsets of the variable data. In other words, for disk_out_ref.exo, allow user to only write out Temp.

This must work with the trace recorder.

User Tag: Watney.