One time conversion function to rule them all

[subagonsouth]

          It seems like we have several functions with slightly different names all doing similar conversions. Would it be beneficial to combine these into a single function with keyword argument inputs so that we are just referencing this one function with different parameters as we need them? Just a thought I'm writing down in case this appeals to you, no need to do it here.

def convert_time(my_time_in_whatever_frame, input_frame="TT", output_frame="ET"):
    # Some logic to select your input functions
    ....
    # Vectorize the spiceypy calls
    np.vectorize(func, ...)

Originally posted by @greglucas in #1295 (review)