Improved energy distribution using NeSST

[shimwell]

This PR is an attempt to make use of NeSST to define the neutron spectra. Closes #88

I have a bit more work to do before it is ready, for example I'm going to see if I can combined the Muir spectra for DD and DT neutrons with the openmc.stats.Discrete distribution for the TT neutrons.

going to try combining with this

[shimwell]

This PR has turned into a bit of a big refactoring, sorry for the large diff

On the plus side it does now solve #80 and #37 and touches on addressing Liams comments in #47

[codecov]

Codecov Report

Attention: Patch coverage is 97.19626% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 95.74%. Comparing base (64e4aa1) to head (bbdb65c).
Report is 6 commits behind head on main.

❗ Current head bbdb65c differs from pull request most recent head e860a74. Consider uploading reports for the commit e860a74 to get more accurate results

Files	Patch %	Lines
src/openmc_plasma_source/fuel_types.py	95.12%	4 Missing ⚠️
src/openmc_plasma_source/__init__.py	83.33%	1 Missing ⚠️
src/openmc_plasma_source/tokamak_source.py	98.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
- Coverage   95.96%   95.74%   -0.23%     
==========================================
  Files           7        5       -2     
  Lines         347      235     -112     
==========================================
- Hits          333      225     -108     
+ Misses         14       10       -4     

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[shimwell]

tests are passing locally and on CI
the examples are running locally and on CI

only missing code coverage now

[shimwell]

This PR is currently reducing the package by a reasonable amount of lines of code while also adding functionality.

I've redone the checking of values so that it is more concise as mentioned by @RemDelaporteMathurin

I'm keen to see if this issue can be fixed before we bring in nesst as a dependency

[shimwell]

I just noticed another part of the code that will need changing to support DD and DT reactions. It looks like this part is only considering DT reactions

openmc-plasma-source/src/openmc_plasma_source/tokamak_source.py

Line 416 in 64e4aa1

return ion_density**2 * DT_xs(ion_temperature)

So we might need to add DD reactions

Also I guess the ion_density for DD pairs is also half that of DT pairs

@RemDelaporteMathurin do you remember why this is ion_density**2 here in the code?

[shimwell]

closing in favour of #95