Changes

Add Cerenkov pre-generator (#1108)

* Draft optical step collector
* Draft Cerenkov pre-generator
* Minor improvements; Draft test
* Inline pre-generator
* Update test
* Draft OpticalTestBase class
* Refactor Cerenkov test using PreGenerator and OpticalTestBase classes
* Remove unused code
* Clean up; minor refactoring
* Address Seth's comments for the Cerenkov test
* Remove OpticalStepCollector
* Calculate number of photons per length during PreGenerator construction
* Add below optical threshold test to the Cerenkov pre-generator test
* Simplify CerenkovPreGenerator constructor parameters
* Refactor to use views
  - Maximize use of Views to collect data
  - Unavailable data from views is cached in the OpticalPreGenStepData placeholder
* Address comments