Support expanding elements in MCNP by weight percent

[paulromano]

Description

Current, the PluginMCNP class allows a user to expand elements that appear in a material definition according to their naturally occurring isotopes. However, this only works when the corresponding fraction is given as an atom percent. This PR extends this capability to work for weight fractions (specified as negative numbers in a MCNP material card).

Note that this required access to atomic masses, which I've pulled in from the AME2020 data file.

Fixes #101

Checklist:

• My code follows the style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• I have updated the CHANGELOG.md file (if applicable)
• I have successfully run examples that may be affected by my changes

[nstauff]

Thanks for this development. do you want to mention this in the documentation? https://watts.readthedocs.io/en/latest/user/plugins.html

[paulromano]

@nstauff Just updated the example in the documentation to use weight fractions