Store atomic mass in ParticleType.

[GuySten]

Description

Atomic mass is a property of particle type so this PR makes atomic mass a property of the new ParticleType.

This makes the atomic mass of a particle more accessible.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• 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)

[paulromano]

Thanks @GuySten -- I think this will come in very handy. However, it would be nice to get this PR updated so that it doesn't change test results (see my comment below)