Add AAVSO-style differential photometry

[mwcraig]

No description provided.

[mwcraig]

Start with chapter 5 of this: https://www.aavso.org/ccd-photometry-guide

particularly the part near the bottom of page 46.

[meyerpa]

need function signature to continue.

[mwcraig]

Ack, sorry!

Feel free to change any of the variable names in what is below or to change the function name.

def aavso_differential_photometry(instrument_mag,
                                  ref_star_intrument_mags,
                                  ref_star_ref_mags):
    """
    Return differential photometry for each star in the table ``instrument_mag``
    assuming that the rows indicted by ``reference_stars``, with
    magnitudes listed in ``ref_star_mags``.

    Parameters
    ----------

    instrument_mag : array-like
        Instrumental magnitude for each of the sources on which photometry is
        to be performed.
    ref_star_intrument_mags : array-like
        Instrumental magnitude of each of the reference stars.
    ref_star_ref_mags : array-like
        Reference magnitudes of each of the reference stars.

    Returns
    -------

    diff_mag, diff_mag_err : array-like, same shape as ``instrument_mag``
        Computed differential magnitude and error for each star in the
        ``instrument_mag`` array.
    """