Merge branch 'knepley/feature-ts-landau-moments' into 'main'

TS Hamiltonian ex4: Preliminary moment equations

See merge request petsc/petsc!8573

DM: Allow creation of a gradient matrix between spaces
- This is like the mass matrix, but with a gradient on the continuous space
- We use this for conservation laws in moments of particle systems
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DM: Allow creation of a gradient matrix between spaces
- This is like the mass matrix, but with a gradient on the continuous space
- We use this for conservation laws in moments of particle systems
- Add projection to DMSwarm to implement this

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