Add BJD to Primary Header

[bjfultn]

• include a photon-weighted BJD for the entire instrument in the primary header

[pranavp25]

Hi BJ,

I experienced this issue recently. I was pulling the MJD from the primary header, and applying a barycentric correction to it, and found that the time corresponded to the start of the exposure, which was off from the BJD in the CSV files we download from "Plots" in Jump. I talked to Howard, and he told me to write a blurb about it so that it can be put in FAQ in the docs, until the BJD is added to primary header. Here it is, feel free to edit as you see fit:

The MJD in the primary header corresponds to the UT time of the start of the exposure. For L0/L1 files, please be aware of this before applying barycentric correction to the time. In the L2 fits files, The BJD corresponding to the mid-point of the exposure can be found in HDU# 8, under "CCD1BJD" or "CCD2BJD".