Add Schur Complement Preconditioner #282

Job	Run time
Documentation	3m 49s
Julia nightly - macos-latest - aarch64 - pull_request	8m 18s
Julia lts - macos-latest - aarch64 - pull_request	6m 3s
Julia 1 - ubuntu-latest - x64 - pull_request	9m 31s
Julia lts - ubuntu-latest - x64 - pull_request	6m 2s
Julia nightly - windows-latest - x64 - pull_request	13m 27s
Julia nightly - ubuntu-latest - x64 - pull_request	10m 14s
Julia lts - windows-latest - x64 - pull_request	8m 49s
Julia 1 - macos-latest - aarch64 - pull_request	12m 11s
Julia 1 - windows-latest - x64 - pull_request	12m 31s
	1h 30m 55s

Provide feedback