Revert to no corner deposition

Introduced to different methods for masking the sidewall
deposition. One that allows deposition at the corners and one that
doesn't. Reverting back to the default of having no deposition in the
corners. This results in seams along the side of the deposition
profile, but is more physical.

 * introduce `MaskedVariablesCorner` and `MaskedVariableNoCorner`
   class