Merge branch 'feature/backport-suitesparse-test' into 'releases/2.4'

Backport suitesparse test to 2.4

This is a straightforward backport of the new SuiteSparse test from
master by simply cherry-picking all commits that touched either
`FindSuiteSparse.cmake` or `FindUMFPack.cmake`. On top of that, I
removed the deprecation warning from the old UMFPack test and extended
it to automatically populate `SuiteSparse_ROOT` from `UMFPACK_ROOT` if
only the latter is set.

This fixes #14 

See merge request !40