added a mt class in core that can be used as a basic mt object that is user friendly and takes advantage of all existing objects.
fixed bug in z when initializing to calculate resistivity and phase
added a class mt to core to be the basic object the user deals with
fixed but in core.z.Z when computing res_phase if freq is None
fixed bug in modelling/ws3dinv.PlotResponse when setting z_object and z_res_object
Merge branch 'master' into lk.modem
Modem - included functionality for merging periods into the MakeData script - consistent rounding of period values to 5 digits introduced

Signed-off-by: LK <LK@uni>
changed all lst to list in imaging for clarity
fixed bug in occam1d with plotting and added flexibility with startup file. Added flexibility with writing WS_initial file
added module for ModEM response plotting: modemPlotResponse.py

Signed-off-by: LK <LK@uni>