Spherical

[mthran]

Description

This PR introduces several small changes for Spherical<3> and Spherical<4>, namely:

• The distance function from Metric for Spherical<3>' and Spherical<4>' is modified to clip the argument to the range of [-1.0,1.0].
• Implementation of a new function from_versor for Spherical<4>
• More efficient LocalTrial for Spherical<4> based on the exponential mapping of the manifold.

Motivation and context

These changes are largely motivated by my own use of hoomd-rs to simulate points on 2- and 3-spheres.

• The distance function for Spherical<N> needed to be clipped because floating point errors sometimes made antipodal points return a distance of Nan instead of pi.
• The from_versor function lends itself to a natural and efficient representation of 3-sphere points. In practice this is helpful for initializing points randomly on the 3-sphere.

How has this been tested?

This PR includes unit test functions for all the new functions. I have also been using this branch to run production simulations and have verified that the functions work as expected. For example, thermodynamic quantities calculated from the Spherical<4> methods reproduce values given by older published simulation results

Checklist:

• [ X ] I have reviewed the Contributor Guidelines.
• [ X ] I agree with the terms of the hoomd-rs Contributor Agreement.
• [ X ] 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.

[joaander]

Thanks! Please address the few suggestions I made and then we can merge this branch.

[joaander]

Thanks!