[FIXED] a bug where, when you disconnect a load (or gen), the next action cannot be performed
if it modifies the load (or gen), because you "cannot change the value of a disconnected load (or gen)"
[IMPROVED] initialize the underlying "PandaPowerBackend" without numba
[IMPROVED] grid2op import to be more compliant with renaming of uppercased file names
[IMPROVED] decoupling of the PandapowerBackend class and the class "internally" used by LightSimBackend
when loading the grid. This caused some issue, eghttps://github.com/rte-france/Grid2Op/issues/508