Discretize

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

Clean up Tree Mesh imports
- pr 63. commits from lheagy, review from rowanc1

Try / except the import of TreeMesh in Tests. This should remove all dependence that the cython code be properly set up in order to import discretize (continues to be an issue on windows...)

IO: UBC meshes
contributions from: thast, micmitch, review from lheagy
from pr 54

- discretize.TensorMesh.readUBC is now a wrapper for loading either 2D or 3D mesh
- discretize.TensorMesh.readUBC can take care of recognizing if the file is a 2D or 3D UBC mesh, or it can be an input given by the user (arg: *meshdim*)
- the original functions are now *hidden* class function under discretize.TensorMesh._readUBC_3DMesh and discretize.TensorMesh._readUBC_2DMesh
- comments in UBC files are assumed to be marked with "!" in front

utils.Zero: Numpy upgrades
contributions from: lheagy, review and comments from rowanc1, jcapriot
from pr 61
- Updates to `Identity()` and `Zero()` so that they are consistent with numpy upgrades in numpy=1.13.0 (https://github.com/charris/numpy/blob/master/doc/source/reference/arrays.classes.rst)
- use scipy.sparse.dia_matrix for spzeros (closed 60)

Noteworthy Changes
- use a `try` / `except` pair when importing the TreeMesh in the main init
- have efficiency warnings state that the user should use `python setup.py install` if cython code is not there in the interp utils
- use a `try` / `except` in the setup.py when importing numpy to tell the user to install numpy if it fails.

Minor Changes
- add codacy, quantified code badges to docs
- pinning to sphinx==1.5.6 for sphinx gallery to build. This is stated in 57 and will be released when upstream changes are made