[petsc-dev] Promote MatShellSetOperation() and MatShellSetContext() to Mat

[Jed Brown]

We already semi-sanction using MatShellSetOperation() to provide different
implementations for matrices that are not of type MATSHELL. An example of
this is having a matrix-free way to apply the action of a MatNest in one
pass over the mesh.

The problem is that one can't call MatShellGetContext() if given a matrix of
a type that is not MATSHELL. MatShellSetContext() is just silently ignored
and then NULL is returned by MatShellGetContext().

Should we promote these functions to Mat, as in MatSetOperation(),
MatSetContext(), and MatGetContext()?

The alternative is for the user to make their application context a
PetscObject so that it can be composed. I've done this quite a lot, but I
don't think it should be necessary.

Also, this looks like a minor naming inconsistency:

PCShellSetContext
DMSetContext
MatShellSetContext

KSPSetApplicationContext
SNESSetApplicationContext
TSSetApplicationContext
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