Add option to force dimensionless constants

[MilesCranmer]

Feature request from the forums: MilesCranmer/PySR#606

Seems like a good idea to have this, so this PR enables such an option under dimensionless_constants_only, which allows the user to disable the "wildcard" unit behavior.

[github-actions]

Benchmark Results

	master	571f85b...	master/571f85bd6df456...
search/multithreading	16.2 ± 0.39 s	16.9 ± 0.23 s	0.957
search/serial	32.4 ± 0.045 s	32.3 ± 0.012 s	1
utils/best_of_sample	0.761 ± 0.22 μs	0.742 ± 0.23 μs	1.03
utils/check_constraints_x10	11.3 ± 2.9 μs	11.6 ± 2.9 μs	0.979
utils/compute_complexity_x10/Float64	2.19 ± 0.12 μs	2.21 ± 0.11 μs	0.991
utils/compute_complexity_x10/Int64	2.18 ± 0.11 μs	2.19 ± 0.11 μs	0.995
utils/compute_complexity_x10/nothing	1.46 ± 0.09 μs	1.44 ± 0.091 μs	1.01
utils/optimize_constants_x10	29.6 ± 6.7 ms	30.5 ± 6.7 ms	0.97
time_to_load	0.87 ± 0.0021 s	0.877 ± 0.0098 s	0.991

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).