Run constant current simulations

Address #16

Add parameters for constant current simulations and evaluate the
current in a notebook.
New `ExtemeFill2DMesh` class

Address #16

If the mesh is changed from an annular representation to a via, the
`build_mesh` raises an exception. The mesh build needs to be fixed so
that it works for annular and via shapes. As a precursor to this the
mesh functions were refactored into a class to make future subclassing
easier.

 * include sumatra setup instructions in README.md

 * `import scipy` and `import scipy.optimize` as this stopped working

 * move mesh building functions to one ExtremeFill2DMesh class
Fix `ExtemeFill2DMesh` for via meshes

Address #16

The `ExtemeFill2DMesh` has been modified to allow via meshes
corresponding to `rinner = 0`. The mesh inner padding is modified to
be the minimum of "rinner - 0" or the padding depth.

 * implemented a test case with rinner = 0
Fix `ExtremeFill2DMesh` docstrings

Address #16

Expand docstrings in mesh class
Clean up the `ExtremeFill2DMesh` docs

Address #16
Modify viewers for new feature shape

Address #16

Modify viewers to account for larger features and via feature. The
viewers are very badly written and need to be simplified.
Mask cupric diffusion to stop deposition

Address #16

Stop deposition on the side walls and upper surface by masking the
cupric diffusion coefficient close to the walls.

 * introduced `masked_harmonic` variable to set the cupric diffusion
   coefficient to zero to stop deposition in given regions

 * reduced current as the default system was struggling to reach
   prescribed current

 * use the constant current parameters in the constant current test
Use `>=` instead of `>` for harmonic

Address #16

Use `>=` instead of `>` for the harmonic variable since values of
phi==0 are deemed to be in the electrolyte.
Merge remote branch 'origin/issue16-prototype_simulations' into issue16-prototype_simulations
Update image of constant current deposition
Enable deposition in corners

Address #16

Corners were being trapped. This change should stop that occuring.
Notebook with no side wall deposition

Fix #16

The no side wall deposition finally works has demonstrated by image in
the notebook.