Brought test case where obstime kwarg is set out of test_calc_rad_loss()
and into new function, test_calc_rad_loss_obstime().
Changed arrays to astropy Quantities in this new function and
time arrays from numpy datetime64 to datetime objects.
Deleted definition of obstime in test_calc_rad_loss_nokwargs().
Renamed test_calc_rad_loss() which now only tests output when obstime
and cumulative kwargs are set, to test_calc_rad_loss_cumulative().

Changed arrays to astropy Quantities and obstime to array of datetime
objects.

Changed expected cumulartive array as the values given by calc_rad_loss()
are slightly different now that scipy.integrate.cumtrapz() is used.