Build PCMS without MeshFields library

[jacobmerson]

To limit the number of dependencies required for PCMS, we should enable building without MeshFields.

Items that need to be resolved:

1. Add a Omega_h adapter type that can handle order 0 and order 1 Lagrange fields. For the linear fields we can simply use the barycentric coordinates as our shape functions and this is already built into Omega_h.
2. Gate all calls to MeshFields
3. Add CI test to build with -DPCMS_ENABLE_MESHFIELDS=OFF

[jacobmerson]

This is done in my "pcms-next" work.