Extend Input Parameters for Minimax Grid Test

[AlexBuccheri]

GX-TimeFrequency/test/test_gx_minimax_grid.py currently tests gx_minimax_grid using only one choice of Min/Max transtion energies, specifically (2., 30.). This can be extended to sweep some range of values using pytest's paramterize fixture.

In particular, (0,0), (1, 0), (0, 1) should be tested to check for error-handling.

Very large transition ratio should also be used.