levels: first draft of levels
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
multi level provider: some minor tweaks
tests: adding and fixing tests for AbstractLevel and TimeLevel
Merge branch 'feature/the-big-cleanup' into feature/rewamping-multi-levels
multi_levels: some renames to shorten module names
levels: tweaking AbstractLevel and TimeLevel (also adjusted tests)
adding some basic custom ABCs

(similar to those defined by collections.abc)
Data Structures: AbstractSpatialObject and default Scalar1D
Data Structures: remove Comparable from AbstractSpatialObject
tests: adding tests for Scalar1D data structure
Merge branch 'feature/spatial-object' into feature/rewamping-multi-levels
Level Transitioners: AbstractLevelTransitioner + SpaceInvariantTimeTransitioner
remove FullWeighting and Injection legacy level transitioners
level transition: some tweaks and fixes for SpaceInvariantTimeTransitioner
tests: for SpaceInvariantTimeTransitioner
tests: minor fix for Scalar1D data structure