Description of Boolean enabling Survival Normalization.

Description briefly of functionality and current limitations.

Described Survival Normalization for phase-space sources. Unfortunately there is little description or justification for this out in the general topic literature. Though I have found it to work better in multiple test cases, notably the FOM in areas with middling statistics (~10%).

In term should add capability of normalization for biased general sources.

Code commenting for actual normalization mechanism

Code commenting

Survival normalization pr 3

Remove duplicate description of Surv Norm

Remove duplicate description

Survival Normalization by Default Off

Update text on Survival Normalization, default=false

Better explaining when to use survival normalization

Single variable for Survival Normalization

Use single variable wgt0, and multiply cutoffs here (on the fly), 

rather than having to multply in simulation.cpp.
and then telling the physics modules to get this or that cutoff.

Revert to original, now that cutoffs were multiplied on the fly

Revert with simplified survival normalization

Assign Particle Start Weight

Punctuation

Remove lingering cuttoff variables in particle_data.h

Bracket typo in physics.cpp

Not entering final if statement (russian roulette for not survival normalization)

Commits to address review

multiplication by wgt0 if Survival Normalization on

Pedantic capitalization to match physics_mg.cpp

Reorganize and reorder Boolean flags to account for changes that have occurred in the interim with the recent addition of weight_window_checkpoint_surface and weight_window_checkpoint_collision.

Thus Resolving a conflict in the present PR.

Co-authored-by: Ethan Peterson <[email protected]>

no boolean gating off survival normalization for SSR only

Co-authored-by: Ethan Peterson <[email protected]>

no SSR gating

remover "relevant for only SSR"