simpeg/simpeg: VRM

Core code for VRM problem Here, we add the core functionality for using potential fields to model viscous remanent magnetic responses. The approach has been adapted from the thesis "Forward modeling and inversion of viscous remanent magnetization responses in the time domain" (Cowan, 2016). Details from pr #645 commits from @dccowan review from @lheagy Highlights Model VRM responses on 3D tensor and 3D tree meshes Model VRM responses for a large variety of current waveforms Model VRM responses for a multitude of transmitter types Invert the response to recover a representative physical property for VRM Future work Provide adequate documentation Provide working examples with figures Provide workflow for removing VRM response from TEM data Exampl