EDI files with only tippers are present do not plot

[kkappler]

Expected Behavior

If an edi file does not have impedance, but only tipper, for a station that does not have electric dipoles, the plotter functions crash.

Current Behavior

from mtpy import MT, MTData
md = MTData()
md.add_station(list_of_paths_to_edi_files, survey="survey_name")
p = md.plot_mt_response("survey_name.001")
p = md.plot_phase_tensor_map()

Bascially, it looks like impedance values are being looked for but not found.

Maybe instead of letting Z be None it could be an empty iterable?