Elliptic DG: support modifying boundary data

[nilsvu]

Proposed changes

This is used to solve for different variables in some regions of the domain to absorb singularities.
For example, when solving $-\Delta u = f$ we could define $u=u_R + u_P$ in some region of the domain with a known (often singular) field $u_P$ and solve only for the regular field $u_R$ in this region. So, when receiving boundary data from outside this region, we subtract $u_P$, and when receiving boundary data from inside this region, we add $u_P$. We also add $\Delta u_P$ to the volume fixed sources $f$ inside the regularized region.

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