Fix double overload in LocalAtomSet code when gmx::index is int

Fix double overload in LocalAtomSet code when gmx::index is int

In cases where the int type matches the gmx::index (aka ptrdiff) type
two overloaded constructors in LocalAtomSetData became the same, as well
as the overloaded add method. This is now avoided by an enable_if
clause.

Change-Id: I139c7feff141928f71c71d6f6db3ee1bffe325b7