cleanup: moved and renamed solver cores -> integrators
cleanup: integrators: moved sdc into own submodule
cleanup: integrators: module init tweaks
Merge branch 'feature/the-big-cleanup' into feature/rewamping-multi-levels

Signed-off-by: Torbjörn Klatt <t.klatt@fz-juelich.de>
cleanup: integrators: further renamed integrators
levels: glueing integrators to time levels
multi level provider: starting rewrite of MultilevelProvider

TODO: add transitioners
tests: first dummy test for abstract levels

TODO: probably need to remove hierarchy position from levels