H3ppy

* 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

* Fixed T < 100 K error reporting in fit()
* Increased the default number of default iterations in fit() - was seeing it leaving the function too early.

* Fixed keyword argument bug. Changed '-' to '_' to make valid keywords (thanks to Emma for finding this bug!)
* Added add_noise() - untested as of yet

* Fixed an exponent error in the temperature derivative of the H3+ partition function, dQdT
* Removed negative line width sanity check - failed when fitting polynomial expressions of the sigma
* Now sorting the get_results() and print_vars() output
* Added get_rest_wavelength() which returns the offset wavelength scale at H3+ rest wavelengths