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.