[bugfix] Make static members constexpr
static const
members need a definition at namespace
scope if ODR-used. Here the static member dim
is
passed to a stream which may (or may not) be ODR-use.
The fact that it works so far is a lucky conincidence
since DUNE_THROW
happens to be a non ODR-use.
Since C++17 static constexpr
members are implicitly
inline, they don't need a namespace scope definition
even if ODR-used.