Allow user specify ra,dec for fits input

[tsunhopang]

@mcoughlin Adding the missed functionality of letting user specify ra and dec in the prior file when using fits input

MAP iota prior with the gw170817.fits:

iota prior with ra = 3.44616, and dec = -0.408084:

[mcoughlin]

LGTM

Maybe should note somewhere the radian bit? Or specify somehow?

[tsunhopang]

It is input via the prior file, from which the ra and dec are always assumed to be radian (the same as in bilby)

[mcoughlin]

Great thanks @tsunhopang

[tsunhopang]

@bfhealy could you check of the failing test for training?

[bfhealy]

@tsunhopang Sure, looks like it has something to do with this error:

TypeError: Could not locate function 'mse'. Make sure custom classes are decorated with `@keras.saving.register_keras_serializable()`. Full object config: {'module': 'keras.metrics', 'class_name': 'function', 'config': 'mse', 'registered_name': 'mse'}

I'll look into it some more, probably related to the recent tensorflow 2.16.1 release.

[bfhealy]

@tsunhopang I propose to merge #334 to avoid this error for now. #335 is a reminder to refactor the code the address this more thoroughly in the future.