[spaces.cg.pdelab] update copy behaviour, refs #28
[spaces] replace manual grid walks
[products/assembler] unify tbb usage
[assembler] moves tbb stuff to walker completely
[test.products] adds smp testing for assemblable products
[test] makes smp checks actually do what was intended

previously second assembly call was practically noop
[test] prepare spaces_cg for smp checks
[test.products] get per-dim number of gridlements via config

requires dune-stuff udpate
[df] ConstDiscreteFunctions keep a space per thread now
PerThreadValue has moved headers
[products] fix finalizing LocalizableBase
[df] manual DiscreteFunction move ctor -> explicitly defaulted
[assembler.wrapper] adds syncing in matrix constraints
[spaces] fixes compile without dune-fem present
[tests.discretizations] fixes a few issues with missing alugrid/dune-fem

disables studies when fem is missing
guards expected results when alugrid missing
removes never called current_grid_width functions