Add conversion from BJD to MJD

[guynir42]

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.