Add get_solvers function for PFmethods

[awig]

Should return what types of optsolvers are available, including checking those dependent on the environment/installed dependencies (such as ipopt).

possible implementations:

at class level:

gridopt.power_flow.ACPF.get_solvers()
returns ['augl', 'nr', 'inlp', 'ipopt' (maybe)]

or at module level:

gridopt.power_flow.get_solvers()
returns {'ACPF' : ['augl', 'nr', 'inlp', 'ipopt' (maybe)],
'DCPF' : None,
...}

@ttinoco do you think both module and class level functions would be useful?