Merge branch 'tisaac/feature-lmvm-rework' into 'main'

MatLMVM rework

See merge request petsc/petsc!6401

Mat: Improve the usability of constantdiagonal and diagonal matrices

Mat_ConstantDiagonal: Add MatConstantDiagonalGetConstant() and implement conjugate and transpose

Mat_Diagonal: Add missing setra

Mat: Improve the usability of constantdiagonal and diagonal matrices

Mat_ConstantDiagonal: Add MatConstantDiagonalGetConstant() and implement conjugate and transpose

Mat_Diagonal: Add missing setrandom, conjugate, and transpose methods

MatView: Don't print MatInfo for constantdiagonal or diagonal matrices

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Merge branch 'tisaac/feature-mat-vectordiagonal' into 'main'

Add MATDIAGONAL and PCMatSetApplyOperation()

See merge request petsc/petsc!6487

Add `MATDIAGONAL` and `PCMatSetApplyOperation()`

`MATDIAGONAL` (`MatCreateDiagonal()`) is an interface for
manipulating a diagonal matrix as a `Vec`, in particular supporting
no-copy modification of

Add `MATDIAGONAL` and `PCMatSetApplyOperation()`

`MATDIAGONAL` (`MatCreateDiagonal()`) is an interface for
manipulating a diagonal matrix as a `Vec`, in particular supporting
no-copy modification of the diagonal and inverse diagonal via
`MatDiagonalGetDiagonal()`, `MatDiagonalRestoreDiagonal()`,
`MatDiagonalGetInverseDiagonal()`, and
`MatDiagonalRestoreInverseDiagonal`().

`PCMatSetApplyOperation()` and `PCMatGetApplyOperation()`
support using all the linear operators currently defined by a `Mat`
(`MatMult()`, `MatMultTranspose()`, `MatMultHermitianTranspose()`,
`MatSolve()`, `MatSolveTranspose()`) as the operation of `PCApply()`
for a `PCMAT`. The advantage over a `PCSHELL` is that the other
operations (`PCApplyTranspose()`, `PCMatApply()`) are automatically
inferred.

These tools help make it easier to route operations like
`VecPointwiseMult()` and `VecPointwiseDivide()` through
`KSPSolve()` to avoid handling diagonal matrices as a special
case (a pattern that is going to be removed from `MATLMVM`).

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