Remove constructor for FV<K1,n> from FV<K2,m> with n!=m

This patch removes the constructor from the overload set.
Before the constructor was there but failed to compile
due to the static assertion.

If you have methods f(K) and f(FieldVector<K,n>) with
n>1 the overload was ambigous when calling f() with
a FieldVector<K,1> because there is a conversion
from this to FieldVector<K,n> - even if it does not
compile due to the failing static assertion.

Now this is avoided using SFINAE. This fixes fs#1024. 

[[Imported from SVN: r6680]]