Add a helper node for testing

[jeremykubica]

While I was testing #162, I wanted a node where I could manually specify which values to return. This provides that and will hopefully make future tests easier to write.

I also renamed the rand_nodes directory to math_nodes for clarity and moved the SingleValueNode there (from base_models.py)

[github-actions]

Before [745e20c]	After [daf633a]	Ratio	Benchmark (Parameter)
failed	1.54±0.1ms	n/a	benchmarks.TimeSuite.time_apply_passbands
failed	4.65±1ms	n/a	benchmarks.TimeSuite.time_chained_evaluate
failed	10.4±2ms	n/a	benchmarks.TimeSuite.time_evaluate_salt3_model
failed	5.57±0.4ms	n/a	benchmarks.TimeSuite.time_evaluate_salt3_passbands
failed	1.99±1ms	n/a	benchmarks.TimeSuite.time_fnu_to_flam
failed	9.11±0.1ms	n/a	benchmarks.TimeSuite.time_load_passbands
failed	32.0±0.2μs	n/a	benchmarks.TimeSuite.time_make_new_salt3_model
failed	1.35±0.02s	n/a	benchmarks.TimeSuite.time_make_x1_from_hostmass
failed	125±2μs	n/a	benchmarks.TimeSuite.time_sample_x0_from_distmod
failed	15.9±0.2μs	n/a	benchmarks.TimeSuite.time_sample_x1_from_hostmass

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