Improve DirectSolver class names (#155) #1518

Job	Run time
clang-format	56s
install-and-build (USE_MUMPS=false)	2m 51s
install-and-build (USE_MUMPS=true)	5m 17s
run-unit-test (false)	2m 37s
run-unit-test (true)	2m 57s
codecov	10s
	14m 48s

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