Deleted some unnecessary error handling in rad_loss_rate().
Edited test_rad_loss_rate unit test to give 100% test coverage of
rad_loss_rate().
Changed a type of error raised in calc_rad_loss() when obstime input has
incorrect number of elements.
Edited test_rad_loss_rate unit test to give 100% test coverage of
calc_rad_loss().
Edited test_xray_luminosity unit test to give 100% test coverage of
xray_luminosity().
Fixed bug in checking length of inputs in goes_lx() and included
conversion of obstime to numpar datetime64 array so other valid input
type are supported.
Moved check that correct exception is raised when only one time is input
to _time_steps from test_goes_lx() to test__time_steps().