NumPy Build & Bug Fixes

• commits from: @jcapriot, @prisae, @thast
• reviews from: @jcapriot, @lheagy

summary of changes

This is a patch to fix a few bugs that have popped up over the last few months, as well as a single function addition to output only the model for a TreeMesh.

Also we have dropped testing of discretize on Python 2.7 as it is officially in end-of-life stage (as of January 1st, 2020).

• From #191:
  Enables discretize to be installed on a system without NumPy present beforehand (for example when pip installing discretize).
  It also contains a small update to the curvilinear mesh view that updates that function to be in line with the other plotImage functions.

• From #189:
  Bugfix. Slicer failed if xslice was provided without a yslice, because of wrong comparison.
  Before, if you provided a homogenous model, it looked like this:
  
  So the colour in the xy- and xz-plots is wrong. As soon as you scroll, the colour in the yz-plot will change to the wrong colour too.
  With the bug-fix, they all get the right colour, and will remain correct if scrolling:
  
  The bug only affects homogeneous fullspaces, which caused an issue with the range used by colorbar().

• From #187:
  We can output models through the writeUBC method of the TreeMesh. However, we might want, for symmetry with TensorMesh, also have the writeModelUBC function, which will only write out the model.