Add find_duplicate_points Internal API

[isVoid]

Description

This PR adds combine_duplicate_points API. Given a multipoints range, this API sets a stencil range for each of the duplicate points in the multipoint range.

closes #814

A second introduction in this PR is make_multipoint_array, a test utility to create an owning object with pure initializer_list syntax for convenience construction. Example usage:

using P = vec_2d<float>;
auto multipoints = make_multipoint_array({
   {P{0.0, 1.0}, P{5.0, 7.0}},
   {},
   {P{3.0, 3.0}}
});

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[harrism]

Looks good. My only concern is with the naming.

[harrism]

What do you mean by "stores as stencil"? Do you mean an array of flags indicating which points are duplicates?

[isVoid]

Doc updated as a result of addressing: #815 (comment)

[harrism]

Just thinking about host / device here. Is this really intended to work on the host? What if it is called from host when the iterator points to device memory?

[isVoid]

This can work on host, if all vectors are host vectors. Although I don't really see a use case at the moment. Up till now I mostly rely on developer's prudence to not access device memory from host.

[robertmaynard]

Approving CMake changes

[harrism]

@gpucibot merge