Fully implement Astropy units

[jgieseler]

Adjust code so that Astropy units are used wherever possible. This is already included in recent code changes like the following:

solarmach/solarmach/__init__.py

Line 262 in 2c635da

alpha = math.degrees(omega * (radius-target_solar_radius*aconst.R_sun).to(u.km).value / vsw)

But in this example, the solar wind speed vsw is still fixed to km/s. Fully incorporating units would help to avoid possible problems.