Add conversion from BJD to MJD

Some observatories (e.g., TESS) will give you the observation timestamp in Barycentric Julian Date (BJD), while we usually want to put all the timestamps into MJD.
The astropy Time object for now accepts only MJD but there's a correction factor that needs to be added based on the position of Earth and moon, e.g., https://mail.python.org/pipermail/astropy/2014-April/002844.html

For now I think the time precision is not critical but if we need really precise timing we must address this.

Start by locating where the package uses the astropy Time object and review the linked Astropy discussion about BJD correction. Done means BJD timestamps can be converted to MJD with the intended Earth and moon correction; the issue names no repository file or test, so those entry points must be identified first.

Written by the indexing model from the issue text.