LA: allow to solve block linear systems efficiently #225

Job	Run time
Setup - Linux	11s
Windows (MSVC) - MPI OFF - Debug OFF	7m 11s
Windows (MSVC) - MPI ON - Debug OFF	9m 12s
Windows (MSVC) - MPI OFF - Debug ON	3m 20s
Windows (MSVC) - MPI ON - Debug ON	4m 21s
Linux (gcc) - MPI ON - Debug ON	18m 9s
Linux (gcc) - MPI ON - Debug OFF	6m 20s
Linux (gcc) - MPI OFF - Debug ON	10m 36s
Linux (gcc) - MPI OFF - Debug OFF	4m 29s
Linux (clang) - MPI ON - Debug OFF	5m 46s
	1h 9m 35s

Provide feedback