excluding std<0 regions

[themiyan]

In find_excluded_lbrange, minwave=3 input cannot be passed through as keyword in fit_spec.

So when there are masked values but not consecutive, i.e. std<=0, find_excluded_lbrange function is triggered. But the minwave determined that lines are not needed to be masked. So an empty array [] is returned.
So in linelist.py`` if exlbrange is not None is False, thus an empty array is passed on to for exlb in exlbs: triggering an error.

A quick fix is to add:

  if excluded_lbrange is not None:
       if len(excluded_lbrange)==0: excluded_lbrange = None

after line 609 in line_fitting.py