H3ppy

Another attempt at fixing missing line lists...

Fixed H2 partition function bug added missing line lists.

* Major optimisation of how the spectra is calculated, removed multiple for loops, which should speed things up significantly. h3ppy now calculates the intensities for all the lines in the line list, rather than just a subset.
* Updated the H2 partition function (Q) fit, valid between 250 and 2000 K
* Included the H2 total emission as just the sum of the individual line intensities

* Fixed problems with the H2 line list data
* Now shortcuts are introduced for setting variables:
- temperature: T
- density: N
- wavelength: wave, w

* Fixed H2 dQdT bug
* Optimised intensity calculation (calculate_line_intensities)
* np.float removed as it is deprecated, changed to float()
* h3p.guess_density made more robust() - hopefully!

* Added experimental support for modelling H2 emission lines