lcfit: add `mandelagol_and_line_fit_magseries`
lcfit: add `mandelagol_and_line_fit_magseries`

analogous to `mandelagol_fit_magseries`, but simultaneously fits a
line + transit model.

this can help avoid underestimating error-bars from "detrending",
because it can let you avoid "detrending" entirely.

a future commit (when I have time/effort) should probably go further,
and implement `mandelagol_and_polynomial_fit_magseries` for arbitrary
order polynomials.

lgbouma/astrobase,lgbouma/astrobase,waqasbhatti/astrobase,lgbouma/astrobase,waqasbhatti/astrobase,waqasbhatti/astrobase