Docs/andrembcosta/phase field fracture

[andrembcosta]

Creating pull request for complete documentation of the Phase-Field Fracture Solver

[joshua-white]

@andrembcosta Can we chat next week before you merge this branch? I have a branch that completely refactors the constitutive classes, including your damage models. It would be easier to add this documentation on that branch, rather than deal with a huge number of merge conflicts.

[rrsettgast]

@andrembcosta Can you get this up to date and ready to merge?

[andrembcosta]

Sure!

[andrembcosta]

I opened #2083 which contains this one, as it became too old. We can get rid of this and just merge #2083.