PETSCFEOPENCL: declare PetscFEGetTabulation_Basic as external

PETSCFEOPENCL: declare PetscFESetUp_Basic as external

Lift changes to defunct dtfe.c from [PR #1074]

Also remove DMFieldGetFEInvariance(), which has
been replaced by DMFieldGetDegree()

PetscSpace: -petscspace_order -> -petscspace_degree

Per Matt's request, a deprecated warning will print for this change.

PetscFE: create directory structure

It's becoming necessary, as we try to do more complex things,
to sharpen the interface between the things in dtfe, and make
extension more approachable. It's tim

PetscFE: create directory structure

It's becoming necessary, as we try to do more complex things,
to sharpen the interface between the things in dtfe, and make
extension more approachable. It's time.

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