Discretize

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

Implement properties in discretize
- from pr 70
- commits from: lheagy, reviews from: rowanc1, fwkoch

Use properties to define the properties of a class that are necessary to completely define the object. This allows us to serialize, save and load meshes (also copy!)

<img width="658" alt="untitled 2017-09-19 10-41-46" src="https://user-images.githubusercontent.com/6361812/30583429-56399bcc-9d27-11e7-9273-b79cf18f21a6.png">

Related issues and discussion
- private properties: aranzgeo/properties191
- arrays with arbitrary shape: aranzgeo/properties192

Implement properties in discretize
- from pr 70
- commits from: lheagy, reviews from: rowanc1, fwkoch

Use properties to define the properties of a class that are necessary to completely define the object. This allows us to serialize, save and load meshes (also copy!)

<img width="658" alt="untitled 2017-09-19 10-41-46" src="https://user-images.githubusercontent.com/6361812/30583429-56399bcc-9d27-11e7-9273-b79cf18f21a6.png">

Related issues and discussion
- private properties: aranzgeo/properties191
- arrays with arbitrary shape: aranzgeo/properties192

originally from pr 67, with commits from thast, lheagy, review from rowanc1
- port over fixes that were made in SimPEG extract core mesh
- add testing on the 1D and 2D extract core mesh util