Merge branch 'barry/2020-10-08/invert-block-diagonal-aij' into 'main'

add MatInvertVariableBlockEnvelope

See merge request petsc/petsc!3544

Implement MatInvertVariableBlockEnvelope() with test example

Commit-type: feature, example
/spend 12h
Reported-by: Olivier Jamond <olivier.jamond@cea.fr>