Removed a lot of the static code in StarData!
More beautiful refactoring.

In StarData, I wrote:
* get_colormag_columns
* get_colorcolor_columns

and the functions
* colormag_axes
* colorcolor_axes
Removed commented-out cruft, added in a couple kwargs.
Reimagined how date_offset works; cleaned up a lot.
Wrote a functioning `multi_lightcurve` code -- proof of concept.
Removed some whitespace in the imports of spread3. Super minor.
Improved/fixed multi_lightcurve, added more documentation overall.