fix: put all operators in vector_utils.h inside namespace

[wdconinc]

Briefly, what does this PR introduce?

This puts all operators inside the namespace edm4eic instead of having part of them in, part of them out. Having some out of the namespace affects other headers included after edm4eic/vector_utils.h, e.g.

In file included from /home/wdconinc/git/EICrecon/src/algorithms/tracking/TrackProjector.cc:30:
/opt/._local/kabrpic7bxoos4p6tucb5wr2sce6oeio/include/edm4eic/vector_utils.h:153:34: error: overloaded 'operator/' must have at least one parameter of class or enumeration type
  153 | template <edm4eic::VectorND V> V operator/(const V& v, const double d) {
      |                                  ^
/opt/._local/kabrpic7bxoos4p6tucb5wr2sce6oeio/include/fmt/core.h:1483:29: note: in instantiation of function template specialization 'operator/<int>' requested here
 1483 | enum { max_packed_args = 62 / packed_arg_bits };
      |                             ^

What kind of change does this PR introduce?

• Bug fix (issue #__)
• New feature (issue #__)
• Documentation update
• Other: __

Please check if this PR fulfills the following:

• Tests for the changes have been added
• Documentation has been added / updated
• Changes have been communicated to collaborators

Does this PR introduce breaking changes? What changes might users need to make to their code?

No.

Does this PR change default behavior?

No. I guess you could be relying on these operators being available outside of the edm4eic namespace... but that's off-label use of this product.

[veprbl]

edm4hep does this too https://github.com/key4hep/EDM4hep/blob/7977dd05311f6b49639b6d2a97373eed03113070/utils/include/edm4hep/utils/vector_utils.h#L270C2-L270C2

[wdconinc]

Yeah, it has another level with utils, but all is in edm4hep at least.

[veprbl]

I don't quite understand the error. It seems like this needs just the explicit keyword in front of each operator definition instead.

If I understand correctly, users may need to put using edm4eic::operator/ after this.

[veprbl]

I see, the VectorND here is a C++ concept. And this only shows up with clang: https://godbolt.org/z/YGa8bW9b8
The explicit keyword doesn't apply since this is not a class. Surprisingly, this works

template <VectorND V, std::floating_point P> V operator/(const V& v, const P d) {
    return v;
}