Add background time selection for ref build

Modified the reference file builder with a toggle switch for adding a
background spectrum for fitting purposes. In addition, I also added another
keyword argument so that you could set the time that you'd like to use for
background subtraction. This time is then converted to an index from the
AIAFile.time array. The AIAFile class saves the index, so that you can check
it later, if necessary.
Added command args for bkg to cal script

The calibration automation script was modified to accept command line
arguements. The first argument '--nobkg' turns off the useage of a MS slice as
a background for fitting. The second argument, '--bkg_time', sets the time
for the background MS slice. The values of these two arguments is saved into
the hdf5 calibration file as attributes of the 'cals' table. That way they can
be queried at a later time.
Let ref spec files have comments.

The referene spectra files can now have comment lines, which must begin with
'#'. This way the ID and location information for the reference spectra can be
saved with the files.