Make Astrometry & SolarSystemShapiro faster

[abhisrkckl]

Avoid creating SkyCoord objects during Astrometry computations. This was partly implemented in #1646 for models with proper motion. This PR does the same thing for cases without proper motion. This has a significant performance advantage.
The tests introduced by #1646 also cover these changes.

In SolarSystemShapiro, avoid doing expensive and unnecessary slices of the TOAs table.

Spun off from #1743, to make testing easier.

[abhisrkckl]

Performance

Here is the profiling code:

from pint.models import get_model_and_toas
from pint.config import examplefile
from pint.residuals import Residuals

m, t = get_model_and_toas(examplefile("NGC6440E.par.good"), examplefile("NGC6440E.tim"))
res = Residuals(t, m)

%timeit res.calc_phase_resids()

Before: 13.3 ms ± 45 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

After: 5.5 ms ± 33.4 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

[dlakaplan]

I think this looks good. Is any explicit new testing required, or is the fact that all of the usual tests passed here sufficient (I suspect the latter since you haven't yet changed the precision)?

Any changes wrt skipping the SkyCoord creation that need tests?

[abhisrkckl]

I think the existing tests are sufficient.

[dlakaplan]

So I can merge?

[abhisrkckl]

Yes.