grounded source implementation for octree

[sgkang]

• LineCurrent class used for time-domain EM code needs to be extended to FDEM problem and Octree Meshes.

• This basically requires to extend getSourceTermLineCurrentPolygon in CurrentUtils.py

• May need some help from @jcapriot

[micmitch]

I also need to start working with grounded sources on the octree mesh soon. Not sure how much help I will be with the octree side of things but I'm happy to try and help.

[jcapriot]

My first inclination of this, is that it shouldn't be too much additional work, as there is already a function to locate the cell (or cells) in a TreeMesh a given point (or list of points) is in.

tree_ext.pyx

Each cell can then be queried for it's node indices, I could also expose the cell's edge/face indices to python as well if that is needed.

[jcapriot]

The source terms for LineCurrents now all support TreeMesh's