Discretize

Cell grad operators for TreeMesh inversion
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- from pr 72
- commits from: fourndo, micmitch, sgkang, lheagy
- review from: rowanc1, lheagy, jiajiasun

- First pass implementation for stencil operators
Initial implementation without hanging faces

- Second implementation with hanging faces

**Smoother gradients on Octree change.**

- Create gradient operators with dimensions for PDE problems.
- Add unit tests for operators. Thanks micmitch and sgkang

Solve DC problem in mine tunnels.
Tree mesh examples will follow shortly on SimPEG ... stay tuned.

- commits from sgkang
- review from rowanc1, lheagy
- originally from pr 88 and discussed in issue 87

create `aveCCV2F`, which averages a cell centered vector to faces and is useful for self-potential problems

From pr: 86
Commits from: lheagy
Review from: thast, rowanc1

add the ability to mask streamlines if the amplitude is below a given threshold (`stream_threshold`).


From pr 84
- commits from thast
- review from lheagy

add IO functionalities to write DCIP2D mesh and models
- the `MeshIO` functions `writeUBC` and `writeModelUBC` are now compatible with 2D meshes and models.

- We now make sure that mesh and model get saved at the same place by giving the option to specify the folder outside of the filename (before the folder was given by the `dict.keys`, not very practical)

- The upgrade is backward compatible

New mesh property: h_gridded
- From pr 77
- commits from: dccowan, lheagy
- review from: lheagy, rowanc1

This function works for TensorMesh and TreeMesh types. Much like gridCC, h_gridded returns [hx,hy,hz] as a nCxdim numpy array. This function is important for multiple potential fields problems (Mag, VRM, grav) in 3D.

Changes from 80
- `properties >=0.3.6b0` is required for running this version of discretize
- `properties` and `vectormath` are installed separately so versions can be managed explicitly