Make Astrometry faster & avoid long double operations where possible

[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.

Also, avoid long double operations where possible. This is done by including a new column tdbfloat in the TOAs table. This is the same thing as tdbld, but reduced to float64 precision.

Includes some refactorings suggested by sourcery.

[abhisrkckl]

Profiling test
(using the NGC6440E example dataset.)

Before

After

i.e., a 48% reduction in execution time.

[dlakaplan]

Have you verified that going to float instead of long double doesn't change precision?

[abhisrkckl]

Closing this because the effects of the two separate changes are difficult to test together.
I'll make separate PRs for each.