Preliminary color evolution comparison

[djperrefort]

This PR adds code for comparing the color evolution of CMFGEN models against CSP DR3. It also includes figures of preliminary results. The relevant issue is #6.

Major changes include:

1. Further development of the analysis.lc_colors module
2. The addition of a command line interace
3. New plots in notebooks/gaussian_regression_colors.ipynb
4. Tabulated residuals in results/color_evolution/*.ecsv

Unfinished work includes:

1. I am currently skipping targets without a CSP published t0 value or E(B-V) value. In the future, we can fit for and / or look up these values in other literature sources.
2. The error values returned by the Gaussian regression are too small. A "correct" (a word I use loosely) error implementation would involve resampling and looking at the variation in the Gaussian fits.
3. I'm not sure what the term "reduced chi-squared" would mean for a Gaussian regression since the definition of the DOF is unclear. Comparing the chi-squared values between targets also doesn't make sense, since the chi-squared values are integrated over different phase ranges. This leaves me unclear on what a reasonable approach would be. In light of these issues and issue 2, I am currently considering the integrated residuals normalized by the phase range in notebooks/gaussian_regression_colors.ipynb.

[djperrefort]

This PR was reviewed offline and approved pending some edits:

Requested changes:

• [x] Add a secondary plot of light curve regression fits in magnitude space
• [x] Increase the resolution of the x-axis for the regression fits
• [x] Make sure colors are red - blue
• [x] Put the number of targets in each histogram

Things slated for future work:

1. A plot of delta color over 15 days for the model vs the data.
2. Plot chisquared for two fixed phase ranges (-10 to 15 and something larger)