Failure of convergence when checked against linear solver(collisional damping)

[shyams2]

Commit Hash : 9518589

Issue :

• The convergence tests for the collisional damping show a convergence rate of N^{-1}, while the expected convergence rate is N^{-2}.
• When the results of the nonlinear solver are checked against the earlier linear theory code which evolved a single mode the convergence test passes. This indicates that something is wrong with the current version of the linear theory code, where we obtain the modes to evolve by performing FFTs.
• Need to find what edge case is being missed out over here in the unit tests of the linear solver.