Wrap discrete_gradient in Python

[mscroggs]

Resolves #3022

[bleyerj]

The lines

dolfinx/python/dolfinx/fem/petsc.py

Lines 785 to 791 in 26f2067

	The discrete gradient operator A interpolates the gradient of
	a Lagrange finite element function into a Nedelec (first kind)
	space.
	Args:
	space0: Lagrange space to interpolate the gradient from
	space1: Nedelec space to interpolate into

seem to suggest that this is made for Lagrange -> Nedelec only. Is this really the case?
I was at some point interested in having a discrete gradient operator from Lagrange to DG for instance, or a Quadrature space.

[michalhabera]

The lines

dolfinx/python/dolfinx/fem/petsc.py

Lines 785 to 791 in 26f2067

	The discrete gradient operator A interpolates the gradient of
	a Lagrange finite element function into a Nedelec (first kind)
	space.
	Args:
	space0: Lagrange space to interpolate the gradient from
	space1: Nedelec space to interpolate into

seem to suggest that this is made for Lagrange -> Nedelec only. Is this really the case?
I was at some point interested in having a discrete gradient operator from Lagrange to DG for instance, or a Quadrature space.

The code makes assumptions that target space maps using covariant Piola, which would fail if you use DG. Just note that you can always build these discrete operators using dolfinx.Expression that contains ufl.TrialFunction on the input space. Assembly of these expressions into PETSc/scipy matrix is not implemented yet, but you can easily write a custom (Numba) loop that does that. I presented this in my FEniCS2021 talk, can find the snippets if you want.