+ manually added some constants for magic numbers

Using `from_string` and `to_string` in user code inside a
`namespace internal` failed, since our variants in
 `Dune::XT::Common::internal` had the same names as the one in
`Dune::XT::Common`, but different signatures. This should fix that issue.

refs #62

also harmonizes from_string arg passing

...so I can to_string Yaspgrid indices