Several fixes for lightcurve

While doing a folded lightcurve of a PLSuperExpCutoff model I found several bugs. This PR makes these changes:

* Compute `gTHIndex,TgrIndex,ArrowIndex` for folded lightcurves so that they can be plotted (l.318 of lightcurve.py). Previously they were not computed and plotting failed when they were not defined.
* Find whether the spectral index is stored in the parameter 'Index' (PowerLaw and PowerLaw2) or 'Index1' (PLSuperExpCutoff).
* Find whether Cutoff is computed for our source, and add plots and WritetoAscii for Cutoff and Cutoff errors.
* Fix the Index/Index1 problem also in fitmaker.py for the FrozenSpectralIndex so that indices can be frozen for PowerLaw and PLSuperExpCutoff (Cutoff however has to be fixed manually).