Fixe var vs. sigma error in speccal.py, so that errors are correctly calculated.

In this code block within speccal:
```
       error=False
       try:
           if obs_spectra.var is not None: error=True
       except:
           error=False
```
the `if` statement was incorrect, as there is
no `.var` extension on the spectra types.

Changed to:
```
       error=False
       try:
           if obs_spectra.sigma is not None: error=True
       except:
           error=False
```

Also cleaned up typos in the comments describing the various parts of
calfunc.  Speccal is now fully compliant with UW_speccal.py.

--Ben Brown, 11/26/13, noon:20pm