RMS Map basemap image + shapefile creation

[brenmous]

• utils.plot_geotiff_imshow now has a function for plotting a geotiff on a pre-existing matplotlib axes. This can be used to provide geotiffs as background images for phase tensor and RMS maps
• Phase tensors and RMS maps can also be written as shapefiles
  • utils.shapefiles_creator handles plotting plotting phase tensors
  • RMS shapefile creation is a method of PlotRMSMap
• Can select periods for phase tensor and RMS map plotting by providing values in seconds
• Can select between plotting impedance, tippers or both on RMS map

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[brenmous]

Thanks Fei!