adding new physics algorithm: center of mass

[santiditomas]

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[cclauss]

This looks super cool! Nice addition. Please consider adding either namedtuples or dataclasses for Particle and Coord3D (please change those names to suit your use case). This would make the function signature:

def center_of_mass(particles: list[Particle]) -> Coord3D:

Here is a bit of code for creating and using namedtuples:

>>> from collections import namedtuple
>>> Particle = namedtuple("Particle", "x y z mass")
>>> p = Particle(1, 2, 3, 4)
>>> p
Particle(x=1, y=2, z=3, mass=4)
>>> p.mass
4

>>> total_mass = sum(particle.mass for particle in particles)

>>> Coord3D =  namedtuple("Coord3D",  "x y z")
>>> Coord3D(6, 7, 8)
Coord3D(x=6, y=7, z=8)

[cclauss]

I went back to collections.namedtuple(). Thanks for doing this.