Spherical cell list

[mthran]

Description

This PR introduces methods and traits necessary to implement cell lists and ParallelSweep for Spherical simulations:

• A new struct SphericalVecCell is implemented whose structure is almost identical to the already existing VecCell. Most of the methods and functionality are identical because Spherical<N> is embedded in N-dimensional Euclidean space. The search radii for SphericalVecCell may be specified either using the Spherical geodesic metric or through the Euclidean metric in the embedding space.
• Some structs and methods in vec_cell.rs which were previously private have been promoted to pub(crate) so as to avoid duplicate code in spherical_vec_cell.rs.
• A new boundary ClosedSpherical<N> which implements Cover<Spherical<N>> is introduced. This is distinct from the pre-existing Closed<T> boundary because the latter implements methods such as Scale and Volume which are irrelevant and potentially misleading in Spherical<N> simulations.

Motivation and context

This PR significantly improves the speed of Spherical<N> simulations, which previously were too slow to run production simulations.

How has this been tested?

spherical_vec_cell.rs contains unit tests for the methods which are unique to itself. I have also run simulations for Spherical<3> and Spherical<4> using the new methods and verified that they are consistent with older simulations using AllPairs.

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the hoomd-rs Contributor Agreement.
• My name is on the list of contributors (doc/src/credits.md) in the pull request source branch.
• I have summarized these changes in release-notes.md following the established format.