Fixing models

Based on https://github.com/astropy/specutils/pull/92. Making models compatible with the latest astropy model framework
Fixes: https://github.com/astropy/specutils/issues/93 partially. 
IRAF specific models will be fixed in https://github.com/astropy/specutils/pull/86.
@wkerzendorf @nhmc How does this look?
I'm going to have to examine this a bit more closely, but I think probably this also needs to be reworked.  Under the new interface `__call__` generally shouldn't be implemented by hand.