Add option to skip saving each decision variable in a separate file (…

[josiahjohnston]

…default is to print out all decision variables).

[josiahjohnston]

Partially addresses issue #104

[mfripp]

I like this, but I think the terminology may confuse people. We generally use "--no-something" instead of "--skip-something" in our flags. But also people probably won't know what's meant by "generic outputs".

I would recommend making this parallel with --save-expression[s] (which defaults to none), by creating a --save-variables flag which defaults to all. Then people could turn off all variables with --save variables none and then turn interesting ones back on with --save-variable BuildGen or --save-variables BuildGen BuildStorageEnergy.

A couple of longer-term improvements (don't have to be done now):

• merge --save-expression and --save-variable (unclear to users why one is saved and not the other)
• create groups other than all and none that people can specify (e.g., all per-period vars/exprs, all per-timepoint ones, all one-off values, etc.)