Mn solver

This MR concludes all work regarding the implementation of the MN_SOLVER.

• Choose MN_SOlVER via SOLVER=MN_SOLVER
• Choose max degree via MAX_MOMENT_SOLVER = 2
• Choose entropy functional

Issues that will be resolved in this ticket:

• Functioning MN_Solver for Linesource test case (#8 (closed))
• Corresponding unit and validation tests (#52 (closed))
• Needed options: Entropy choice and otpimizer choice (#53 (closed))
• Write a standard newton type optimizer (#55 (closed) )

Options for the Newton Optimizer:

• NEWTON_FAST_MODE = NO (Skips the optimizer for qudratic entropy)
• NEWTON_ITER = 100 (Max Optimizer iterations)
• NEWTON_EPSILON = 0.01 (Convergence epsilon)
• NEWTON_STEP_SIZE = 0.5 ( newton step size)
• NEWTON_LINE_SEARCH_ITER = 100 (Line Search max iter)

Code restructuring

• Restructure inheritance of SN, PN and MN Solver. (#54)
• Clean up of header files for hopefully faster compilation.