feat: add variable size subregion

[rrsettgast]

We would like to have variable size subregions for arbitrary polyhedral elements. This means no fixed number of nodes/edges/faces per element.

First cut: just create new elements that have ArrayOfArray maps to the nodes/edges/faces.

• create variable size element subregion
• create variable size cell block (maybe already in place)
• import from vtk (maybe already in place)
• write to vtk plot files (maybe already in partially place)
• modify VEM and mimetic FVM to work with variable sizes.

second cut(later): Create new ArrayOfFlexArray type to group elements similar elements. This is also what we want for the regular elements, as it will allow to treat subregions as fixed size or variable size. This way we can have maps at the ElementRegionManager level, and the ElementSubRegion will just take a fixed size slice (or a variable sized slice)