Support additional EOP products (e.g. EOP 14 C04)

[david-macmahon]

The EOP products currently downloaded by EarthOrientation are the "finals" files, which are categorized as "Rapid data and predictions". IERS also provides Bulletin B ("Monthly earth orientation data") and EOP 14 C04 ("Long term earth orientation data"). These products all cover the same sort of Earth orientation parameters, but they have different sources of input data that are fit to models of different complexity so they can and do contain slightly different values for the same parameters on the same date. It would be great if EarthOrientation supported these additional products so users could choose which product(s) they wish to use.

[helgee]

Good idea 👍

Here is an issue from Astropy where some related improvements are discussed: astropy/astropy#9227