Addition of unit tests

[shyams2]

The following test-problems need to be added to the code:

• Stationary shock test
• Heat conduction check(Fourier Law)

The currently existing unit tests need to be integrated into an automated testing framework by the use of pytest. These namely are:

• Check of convergence for the interpolation routines in velocity and configuration space
• Verification of Poisson solvers with a known analytical solution for the input density.(Check order of convergence here as well)
• Automated verification of convergence of test cases generated thus far(landau damping, collisional damping, etc.)(test-problems)
• Test for FDTD solver.