Simplify LBSSolver class

[wdhawkins]

The following issues need to be addresses as part of the refactoring of LBSSolver:

• Current solver classes (LBSSolver, DiscreteOrdinatesSolver, DiffusionDFEMSolver, DiscreteOrdinatesCurvilinearSolver) should be promoted to problems (LBSProblem, DiscreteOrdinatesProblem, etc.). It may make sense to split the base Solver class into separate Solver and Problem classes with field functions as members of the problem class. Classes now located in modules/linear_boltzmann_solvers/executors should be renamed solvers (LBSSteadyStateSolver, NonLinearKEigenSolver, PIKEigenSolver, etc.).
• Convert flux moment vectors to std::vector<std::vector<>> to eliminate complex copy operations (LBSVecOps)
• Use std::swap where appropriate to prevent copying of flux moments, etc.
• Boundary conditions and sources should be passed to the solver via the lbs_solver block in the input, not the lbs_options block. Boundary conditions are addressed by Adding boundary conditions as an optional solver input parameter #723, Add set/clear sources and boundary conditions methods to Python interface #745, Adding volumetric and point sources as optional input parameters for LBSProblem #748, Removing extraneous boundary processing from SetOptions #765.
• Generic groupset objects
• Move grid-level information to more appropriate locations (grid nodal mappings, grid face histograms, etc.)
• Simplify cell-level discretization mappings (unit cell matrices, cell mappings, cell lbsview)

[andrsd]

lbs_curvilinear_solver.h contains DiscreteOrdinatesCurvilinearSolver. I am renaming it to DiscreteOrdinatesCurvilinearProblem. Do you want the file be discrete_ordinates_curvilinear_solver.{h|cc}?

[andrsd]

Also, SweepChunkPwlrz is in lbs_curvilinear_sweep_chunk_pwl.h. Do you want me to rename the file to something like curvilinear_sweep_chunk_pwl.h and rename the class to match the file or match the file name to the class name? Thoughts?