[WIP] Seeded RNG

[drobnyjt]

Thank you for your contribution to rustBCA!

Before submitting this PR, please make sure you have:

• Opened an issue
• Referenced the relevant issue number(s) below
• Provided a description of the changes below
• Ensured all tests pass and added any necessary tests for new code

Fixes #308, #306, #305, #310

Description

This PR adds a seeded RNG (ChaCha8) to make RustBCA deterministic; as a first pass, the seed will be hardcoded - before merge, I'd like to have it be an optional input parameter.

This also rolls in the powf -> powi optimizations (there's a minor performance cost to determinism that is basically completely counteracted by these optimizations) and the Biersack-Varelas stopping power correction factor change.

I've also updated the README doctests to take advantage of the new determinism.

Remaining tasks:

• add ability to specify seed
• investigate feasibility of environment variable seed for python functions
• update docs once code updates finished to include implementation details

Tests

Cargo test passes; using CI to make sure nothing's broken.

[drobnyjt]

Test using libRustBCA; need to compare all example outputs as well.

[drobnyjt]

This line in the github action lets you manually hit a "run workflow" button in the UI

[drobnyjt]

Switched to branch 'rng_seeded'
M       examples/make_input_file_and_run.py
Your branch is up to date with 'origin/rng_seeded'.
...
Processing 1000000 ions...
Initializing with 6 threads...
[00:03:11][########################################][00:00:00] 100%
Finished!
H on B-TiB2-Ti
R_N, R_E: 0.405277, 0.14430882622055832
Y: 0.07703 [at./ion]

H on B (default settings)
Y: 0.0769 [at./ion]
R_N, R_E: (0.3553, 0.13995051092991748)
Switched to branch 'dev'
...
Processing 1000000 ions...
Initializing with 6 threads...
[00:03:15][########################################][00:00:00] 100%
Finished!
H on B-TiB2-Ti
R_N, R_E: 0.405419, 0.14406380591577478
Y: 0.076484 [at./ion]

H on B (default settings)
Y: 0.0757 [at./ion]
R_N, R_E: (0.3523, 0.13869173319541878)

Comparing the rng_seeded branch to dev with make_input_file_and_run.py (modified)