Change Pulsar to run if user has PINT, but not libstempo

[Hazboun6]

Currently the user gets a printed message that says 'ERROR: Must have libstempo package installed!' if the import call for libstempo raises an ImportError exception. There are a number of user types who do need libstempo installed, including those dealing with gamma-ray or x-ray timing data, since libstempo (really TEMPO2) can't deal with these types of TOAs.

It looks like the only change to the working code would need to be here and would just be changing:

    if pint:
        toas = list(filter(lambda x: isinstance(x, toa.TOAs), args))
        model = list(filter(lambda x: isinstance(x, TimingModel), args))

    t2pulsar = list(filter(lambda x: isinstance(x, t2.tempopulsar), args))

to

    if pint:
        toas = list(filter(lambda x: isinstance(x, toa.TOAs), args))
        model = list(filter(lambda x: isinstance(x, TimingModel), args))
    if tempo2:
        t2pulsar = list(filter(lambda x: isinstance(x, t2.tempopulsar), args))

Besides that we'd just add a truthful error message. Something like: raise ValueError('Must have either PINT or libstempo installed.')

Does anyone (@vallis @paulthebaker @stevertaylor ) know of other places in the code that might require libstempo/TEMPO2?

[paulthebaker]

This is a good idea. You should also change the import behavior to give warnings if either is missing.

I don't think PINT or libstempo is used outside of loading Pulsar objects.