Fix OnCell behavior for Quadratic{Flat}
Visual tests + fixes
First attempt at linear bc/extrap for B2
Scrap ExtendInner BC
Infrastructure for padding the equation system

The extra equations from the boundary conditions will not always allow
indexing as we did before, by e.g. doing cm_d = cp_d at the lower boundary
for some BC:s. In order to generalize better, we instead pad the equation
system with one line for each extra equation to allow more general BC specs.

This had the advantage of significantly simplifying some of the code (and,
even better, some of the concepts) required to implement new functionality,
so even though some of the underlying infrastructure is now more complicated
than before, new contributors will hopefully have a lower threshold to start
adding features.
Implement a few more BC:s + whitespace and test fixes