LA: allow to solve block linear systems efficiently #217

Job	Run time
Setup - Linux	13s
Windows (MSVC) - MPI OFF - Debug OFF	6m 52s
Windows (MSVC) - MPI ON - Debug OFF	10m 20s
Windows (MSVC) - MPI OFF - Debug ON	3m 50s
Windows (MSVC) - MPI ON - Debug ON	5m 4s
Linux (gcc) - MPI ON - Debug ON	19m 30s
Linux (gcc) - MPI ON - Debug OFF	6m 37s
Linux (gcc) - MPI OFF - Debug ON	11m 44s
Linux (gcc) - MPI OFF - Debug OFF	4m 52s
Linux (clang) - MPI ON - Debug OFF	6m 59s
	1h 16m 1s

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