Manipulating matrix directly

[francesco-ballarin]

@torhans

Regarding the 2 poissons; what I hope to do is to split a mesh along an internal subdomain boundary and make a discontinuity in the potential.

There are plenty of ways to do that without fiddling with the low level matrices, for instance:

• https://multiphenics.github.io/
• Discontinuity at interface using mixed domains