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.