Merge branch 'nathawani/feature-snes-lowmach-example' into 'master'

Nathawani/feature snes lowmach example

See merge request petsc/petsc!3319

DM: Fix nullspace handling
- Now, nullspace constructors construct a nullspace for the entire DM,
but based on the singularity of an operator connected with the given
field
- remove special handling

DM: Fix nullspace handling
- Now, nullspace constructors construct a nullspace for the entire DM,
but based on the singularity of an operator connected with the given
field
- remove special handling in SNESSetUpMatrices()
- Now all nullspaces are created in DMCreateMatrix()
- Fixed bug in DMSNESCheckJacobian()

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