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

cleanup oldSolution in ProblemInstat, so that it can be used in expressions and allows interpolation during grid changes
4 jobs for issue/problem_instat_oldsolution in 25 minutes and 52 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #8640
debian:10 clang-6-libcpp-17

00:25:52

passed #8639
debian:10 gcc-8-17

00:18:20

passed #8641
debian:9 gcc-6-14

00:19:46

passed #8642
ubuntu:18.04 clang-6-17

00:24:37