Add color chi-squared values and delta color 15

[djperrefort]

Major Changes

• Removes results for normalized residuals of model color evolution
• Adds chi-squared values for color evolution over phase ranges starting at -10, -5, 0, 5, and 10 days and ending at 20 and 50 days (all combinations included in PR)
• Adds tabulated values for change in color over 15 days for all DR3 targets and CMFGEN models
• Splits gaussian_regression_colors.ipynb into lc_gaussian_regression.ipynb and color_evolution.ipynb

Minor Changes

• Updates description of project goals in ReadMe
• Increased test coverate
• Various bug fixes

Notes / requests for reviewer

The files included in the scope of this pull request include the analysis/lc_colors module in addition to the lc_gaussian_regression.ipynb and color_evolution.ipynb notebooks. The module includes the core analysis logic, while the notebooks are mostly plotting code. Note that analysis/lc_colors/_lc_regression.py is a port of logic from avocado and can be ignored in the review.

I am not confident in my shifting of the model time scales to the t0 point of the observed light curves. The code for tabulating the chisquared values (in analysis/lc_colors/_chi_squared.py) definitely got away from me and any feedback concerning simplification is especially appreciated.

[djperrefort]

@wmwv I've addressed your requested changes. Can you take another look?

[djperrefort]

@wmwv I have the football

[djperrefort]

This PR was merged via git but not recognized by GitHub. I'm closing it.