fix: add atan to ValidVector unary operators

[MilesCranmer]

Fixes MilesCranmer/PySR#1068

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	master	35ac3d9...	master / 35ac3d9...
search/multithreading	15.2 ± 0.37 s	14 ± 0.18 s	1.09 ± 0.03
search/serial	34.5 ± 0.31 s	32.6 ± 0.64 s	1.06 ± 0.023
utils/best_of_sample	2.02 ± 0.6 μs	1.85 ± 0.49 μs	1.09 ± 0.43
utils/check_constraints_x10	16.5 ± 4.2 μs	16.7 ± 4.2 μs	0.993 ± 0.36
utils/compute_complexity_x10/Float64	2.15 ± 0.09 μs	2.15 ± 0.09 μs	1 ± 0.059
utils/compute_complexity_x10/Int64	2.08 ± 0.089 μs	2.08 ± 0.09 μs	1 ± 0.061
utils/compute_complexity_x10/nothing	1.54 ± 0.08 μs	1.54 ± 0.09 μs	1 ± 0.078
utils/insert_random_op_x10	5.26 ± 1.7 μs	5.37 ± 1.7 μs	0.979 ± 0.44
utils/next_generation_x100	0.437 ± 0.021 ms	0.436 ± 0.022 ms	1 ± 0.07
utils/optimize_constants_x10	0.0349 ± 0.008 s	0.0335 ± 0.0076 s	1.04 ± 0.33
utils/randomly_rotate_tree_x10	7.99 ± 0.92 μs	7.96 ± 0.89 μs	1 ± 0.16
time_to_load	2.73 ± 0.031 s	2.58 ± 0.029 s	1.06 ± 0.017

Memory benchmarks

	master	35ac3d9...	master / 35ac3d9...
search/multithreading	0.205 G allocs: 53.2 GB	0.206 G allocs: 54.1 GB	0.983
search/serial	0.207 G allocs: 53.8 GB	0.207 G allocs: 53.8 GB	1
utils/best_of_sample	0.038 k allocs: 3.25 kB	0.038 k allocs: 3.25 kB	1
utils/check_constraints_x10	0.034 k allocs: 0.875 kB	0.034 k allocs: 0.875 kB	1
utils/compute_complexity_x10/Float64	0 allocs: 0 B	0 allocs: 0 B
utils/compute_complexity_x10/Int64	0 allocs: 0 B	0 allocs: 0 B
utils/compute_complexity_x10/nothing	0 allocs: 0 B	0 allocs: 0 B
utils/insert_random_op_x10	0.04 k allocs: 1.56 kB	0.045 k allocs: 1.88 kB	0.833
utils/next_generation_x100	4.63 k allocs: 0.276 MB	4.62 k allocs: 0.276 MB	1
utils/optimize_constants_x10	21.6 k allocs: 22.2 MB	21.5 k allocs: 21.3 MB	1.04
utils/randomly_rotate_tree_x10	0.042 k allocs: 1.34 kB	0.042 k allocs: 1.34 kB	1
time_to_load	0.145 k allocs: 11 kB	0.145 k allocs: 11 kB	1