Add user-defined near null space to aggregation-based AMG #84

Job	Run time
Julia 1 - ubuntu-latest - x64 - pull_request	3m 25s
Julia 1 - macos-latest - x64 - pull_request	5m 55s
Julia 1 - windows-latest - x64 - pull_request	4m 33s
	13m 53s

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