Changes

Add neutron elastic scattering physics  (#1120)

* Add neutron and anti-neutron
* Add test data from G4PARTICLEXSDATA
* Add a neutron elastic scattering model (based on the Geant4's CHIPS model), and associated data and classes
* Add a test for neutron elastic scattering model and interactor
* Add a test for NeutronElasticProcess
* Add a test for neutron elastic scattering
* Add a helper class to load neutron elastic cgit commit -m section data (G4PARTICLEXSDATA/neutron/el)
* Add neutron elastic data (NeutronElasticData)
* Add ImportNeutronElasticMap
* Add a neutron_elastic process
* Add the hardwired neutron elastic process and cross section calculation
* Add a minimal implementation of the Lorentz vector
* Clean up a comment block
* Remove the protection for transuranium elements and update for retrieving data from an input file
* Changed variable names (dropping the use of capital Q2) and also updated variables with quantities
* Rename variables containing the capital letter
* Rename variables containing the capital letter
* Change class names from NeutronElasticModel/Executor/Interactor to ChipsNeutronElasticModel/Executor/Interactor
* Change class name from InvariantQ2Sampler to MomentumTransferSampler, and variable name from q2 to q_sq
* Correct the data type in the CELER_VALIDATE message
* Add public for type aliases
* Fix the cross section calculator for the chips model
* Remove unnecessary structs and refactor NeutronElasticData
* Move neutron_mass and neutron_energy to the constructor and initialize them as real_type
* Remove infile.close() which is redundant with the destructor
* Use Real3 defined in geocel/Types.hh and rename v2 to v_sq
* Remove unnecessary make_unit_vector and add an assert for the validity of cos_theta
* Add a brief description for the operator()
* Add a test case with a light nucleus target and fixed the averge rng count
* Change the variable name A to amass
* Fixed CELER_EXPECT for el_id
* Updates for review comments by sethrj
* Use EXPECT_SOFT_EQ for comparing floating point values and use real_type
* Add  for NeutronElastic.test
* Fix (xs) units for chlep ctest
* Updates for code review by amandalund
* Update the precision for s_wave_limit

Add documentation