Merge branch 'issue/problem_instat_oldsolution' into 'develop'

cleanup oldSolution in ProblemInstat, so that it can be used in expressions...

See merge request !95
4 jobs for develop in 38 minutes and 43 seconds (queued for 11 minutes and 16 seconds)
Status Job ID Name Coverage
  Test
passed #8713
debian:10 clang-6-libcpp-17

00:25:59

passed #8712
debian:10 gcc-8-17

00:18:19

passed #8714
debian:9 gcc-6-14

00:19:26

passed #8715
ubuntu:18.04 clang-6-17

00:24:13