Tree functions

• from pr #177
• commits from @jcapriot
• pr started by @micmitch
• review from @lheagy

Updates

• Several of the functions are used to integrate line source terms for FDEM.
• The TreeCell object now exposes it's node/edge/face indices to python, a TreeCell object can be obtained by directly indexing the TreeMesh.
• Added much documentation to the TreeMesh objects and their functions.
• The TreeMesh plotSlice and plotGrid functions now have the same calling conventions as the respective TensorMesh functions