making openmc convertor optional

[py1sl]

How would folk feel about making the openmc tally unit convertor optional either something along the lines of

 try:
        import external_module
    except ImportError:
        external_module = None 

or moving the import into the plot_spectrum_from_tally function?

[shimwell]

Sure we can do this if it helps you in anyway. Feel free to make a PR and we can merge it in.

The openmc_tally_unit_converter is quite minimal, as in it doesn't actually have any dependencies that are not already needed by spectrum plotter. So I can't quite see the benefits myself but if you want it then we will do it

[py1sl]

Spectrum plotter doesn't import openMC directly only via openmc_tally_unit_converter
This change would in future allow spectrum plotter to be extended to other codes / be code agnostic allowing consistent plotting with different codes without needing to install openMC.

[shimwell]

Fair enough. Sometimes I forget there are other codes 😄

Feel free to make a PR and we can merge it in