VMD over a sphere - TDEM

[lheagy]

Time Domain EM sounding over a sphere with a vertical magnetic dipole source and step-off waveform (eventually, it would be fun to play with other waveforms... but that is out-of-scope for now). This can be built from the tutorial notebook showing a TDEM_vmd_sounding_over_sphere.ipynb in the simpeg/em_notebooks:

• sphere in a halfspace - with a single layer above the sphere - on the 2D cylindrical mesh
• user should be able to change
  • depth, radius of sphere
  • conductivity of sphere, halfspace
  • height of source
• for viewing: currents, electric field, magnetic field / flux / db/dt through time

[lheagy]

• also want to be able to view the model
• move time slider to the bottom
• colorbar labels (at least good for defaults)
• "time at" --> "time: "

[lheagy]

• Copy content from: TDEM_HorizontalLoop_LayeredEarth to the VDM app
• use a print statement for the examples that are not valid

[lheagy]

closed with #50 and geoscixyz/em_examples#107