Make FieldVector::size() static
In order to use the
size() method of
FieldVector in compile-time context we made it
constexpr some time ago. Unfortunately this is only partly useful, because one cannot do things like this
template<class V> auto foo(V v) -> std::array<int,v.size()>;
The reason is that v itself is not a constexpr. This can be solved by making the method
static constexpr. Then one could still use it as before, but also in a static context.