Block ID checking for physics

Prevent users from selecting blocks for physics that don't exist in the mesh
Mixed form of peridynamics: separate out the peridynamics pressure field

This new physics is called peridynamics_mixed

First step towards a mixed version. Pulled the peridynamics pressure out
as a separate field with intentions that in future commits this field
could be populated by another physics, or by another function call in
this physics.
Created Continuity Physics

This will be the equation that get solved for pressure in the mixed
form of peridynamics. It's not working for this commit, this is just
a placeholder commit.
Towards coupling schemes

In this commit all of the physics have been changed so that they no
longer have the iteration loops internal to the physics. The
iterations are now executed by the simulation. In this way, various
coupling schemes for different physics can be done. This commit only
sets out the framework for various schemes, none of them have been
implemented. All of the existing tests work by creating a default
scheme that just executes the iteration loops in the original sequence.
Continuity Physics

Solves in nonlocal way int(p + k*theta)dH = 0. This completes the
base implementation of the mixed form of peridynamics. The jacobian
needs some work for the displacement solve. It seems that without the
old pressure terms convergence is pretty bad for finite differencing.
Couping Schemes

Added Jacobi and Gauss Seidel coupling schemes