Refactor PLV handling to be more isolated and dynamic

[matsen]

Right now the PLV management in GPDAG is built in. We should have this factored out into a separate class that only knows about PLVs and likelihoods.

Also, the PLVs for GP are currently numbered in a way that is not so friendly for expanding the number of nodes:

size_t GPDAG::GetPLVIndexStatic(PLVType plv_type, size_t node_count, size_t src_idx) {                                                                                                                
  switch (plv_type) {                                                                                                                                                                                 
    case PLVType::P:                                                                                                                                                                                  
      return src_idx;                                                                                                                                                                                 
    case PLVType::P_HAT:                                                                                                                                                                              
      return node_count + src_idx;                                                                                                                                                                    
    case PLVType::P_HAT_TILDE:                                                                                                                                                                        
      return 2 * node_count + src_idx;                                                                                                                                                                
    case PLVType::R_HAT:                                                                                                                                                                              
      return 3 * node_count + src_idx;                                                                                                                                                                
    case PLVType::R:                                                                                                                                                                                  
      return 4 * node_count + src_idx;                                                                                                                                                                
    case PLVType::R_TILDE:                                                                                                                                                                            
      return 5 * node_count + src_idx;                                                                                                                                                                
    default:                                                                                                                                                                                          
      Failwith("Invalid PLV index requested.");                                                                                                                                                       
  }                                                                                                                                                                                                   
}                                                                                                                                                                                                     

That shouldn't be hard to "transpose".

[davidrich27]

@matsen Should we also take this opportunity to rename the PLV enums? Since the GP paper has moved away from the tilde notation?

[matsen]

@davidrich27 YES!!!

And also make them compliant with the capitalization convention.

[davidrich27]

@matsen Do you want to transpose PLVs from (plv_idx, node_idx) to (node_idx, plv_idx)?

[matsen]

That seemed like a good thing to do if we wanted to be able to expand the number of nodes later, but how we do all of this is up to you!