Refactor library into a single `ligo.p_astro` subpackage

Currently the library installs a few different modules in the ligo namespace (all with p_astro prefix), as well as ligo.data and ligo.tests subpackages. It would be cleaner (IMO) to have all of this under a single ligo.p_astro subpackage namespace. Something like this:

ligo/__init__.py             --> DELETE
ligo/data/                   --> ligo/p_astro/data/
ligo/p_astro.py              --> ligo/p_astro/__init__.py
ligo/p_astro_computation.py  --> ligo/p_astro/computation.py
ligo/p_astro_gstlal_utils.py --> ligo/p_astro/gstlal_utils.py
ligo/p_astro_utils.py        --> ligo/p_astro/utils.py
ligo/tests/                  --> ligo/p_astro/tests/