calc_rad_loss(): aded call to check_download_file() to ensure
chianti_rad_loss.csv exists locally on user's machine.  This also
required added download_dir kwarg to calc_rad_loss() and rad_loss_rate().
Edited doctring of rad_loss_rate().
Some more changes to docstring of rad_loss_rate() and also edited docstring
of calc_rad_loss().
Edited the doctring of xrar_luminosity().
Edited docstring of goes_lx()
Edited doctring of _calc_xraylum().
Edited docstring of _time_intervals().
Edited definition of download and download_dir kwargs in all relevant
docstrings.
Some aesthetic edits to docstrings of temp_em(), goes_chianti_tem(),
_get_goes_chianti_temp(), and _get_goes_chianti_em() to bring them more
in line with PEP8 conventions.