MFPIC Cleanup Ideas

[caleblogemann]

• Consolidate Forward Euler and SSPERK32 into generic explicit runge kutta timestepping
• Look into using transformed coefficients for DGEulerAssembly instead of custom mfem::LinearFormIntegrators
• Split code into sub libraries, possible libraries, utils, output, time_integrators, Euler, PIC
• Create VR ops class/interface
• Reconciling non-Maxwellian PDF evaluations in the euler namespace.