This commit adds the SB interactor and associated kernel method.

  * Initialize `material_view` near where first used

  * `migdal` as `constexpr`, and use integer multiplication to
    prevent float->double conversion

  * Make type aliases `public` and add doxygen mark-up
  * Remove unused `energy_val_max_`

  * Add ElementId class member to interactor and supply during
    instantiation of interactor in CUDA source
  * Remove assertion on minimum energy for interactor
  * Do not apply XS correction for positrons yet in interactor;
    to be integrated in energy distribution sampler
  * Remove `energy_val_min_` member from interactor (handled
    before interactor is constructed); instead, check incoming
    particle energy is sufficient to produce secondary at start
    of `operator()`

Used for sampling angular distributions in pair-production and
bremsstrahlung processes (e.g. Seltzer-Berger and Bethe-Heitler
models).

  * Update class description and other comments
  * Remove photon angular sampler function (using instead common
    TsaiUrbanDistribution)

Sampled electron angle twice. Fixed to sample positron.

  * Use native pointers/refs

\todo Fix issue with min_gamma_energy <= 0

  * Explicitly mark public of type aliases and doxygen

  * EXPECT_VEC_SOFT_EQ for angle comparison with `double` type

  * Store ratio energy [MeV] to mass [MevMass*c^2] instead
    of energy and mass separately

  * `gamma_secondary` -> `secondaries` (for consistency)
  * Assign secondary energy
  * normalize incident particle outgoing direction

  * Finalize 'basic' test
  * Add 'stress' test