Electron energy equation & Plasma mms

[dreamer2368]

Pull after #119 and #120 .

Electron energy equation:

• Work by electron pressure along bulk velocity
  • Adding electron pressure in convective flux was necessary, in order not to lose convergence order.
• Energy sink from electron-impact reactions
• Energy transfer by elastic collisions
• Transport properties necessary for EM-coupling (conductivity or diffusion current) and source term

2D ternary plasma manufactured solution:

• Provide wrappers for exact solution and forcing based on solution name.
• It requires newly added manufactured solutions in the forked MASA repository: https://github.com/dreamer2368/MASA.git
• Newly added solutions are verified for the entire set of governing equations, on 2D-periodic domain.
• Used ConstantTransportProperties: this has the same structure as ArgonMinimalTransport and uses the same subroutines. Only difference is that all the coefficients (viscosity, conductivity, ...) are set to be constant, not depending on collision integrals. This is complemented by other previous test routines collision_integrals.test and argon_minimal.test.