Minor fix to Pandeia notebook

[Skyhawk172]

Pandeia changes:

1. In docstring forcompute_nexp, bracket keyword is for the range of exposures to test, not magnitude. Also, xtold --> xtol.
2. In minimize_scalar , bracket argument is not needed for method = 'bounded'.
3. Minor thing, but I would make the variable names that store the outputs from compute_nexp match those that it returns, to avoid confusion. For example, everywhere in the notebook, we call the ETC output report, but in this particular call, it returns etc . I missed that name change and it took me a little bit to figure it out.
4. If we do if report['scalar']['sn'] < sn: nexp += 1 then we need to run the Pandeia calculation again to get an updated total exposure time, otherwise we’re one exposure time short.