Pair reaction data+kernels and single-pair data unification

[SMijin]

Pair reaction data/kernels and single-pair data unification

Issue: N/A

Spack Spec(s) Tested: spack_default

Environment details: local machine (Docker coding harness)

Documentation PR(Optional): To be done once all binary features are implemented

Summary

• New abstraction layer AbstractReactionData based on templated accessor and argument packs
• Pair reaction data and kernels (see pair checklist below)
• Unification of single and pair reaction data, including composite reaction data

Checklist

• PairReactionData - particle_pair_loop-compatible reaction data
• PairDataCalculator - particle_pair_loop-based data calculator using the above data
• CSData - PairReactionData using a cross-section object to return $\sigma v_r$
• ConstantCrossSection - what it says on the tin...
• PairReactionKernels - particle_pair_loop-compatible reaction kernels (implemented, untested, waiting for SWPMReaction class)
• Unit tests pass (added several composite data tests for pair loops - not exhaustive)
• Updated the documentation.