Create a BasicMathNode that can evaluate expressions given as a string

[jeremykubica]

Creates a new BasicMathNode that will evaluate basic math functions given as a string. Performs a bunch of checks to prevent arbitrary code execution. Also translates math functions into the correct backend (math, numpy, or jax).

This will allow us to write functions nodes (for basic math) as:

x0_node = BasicMathNode(
    "pow(10.0, -0.4 * (distmod - alpha * x1 + beta * c + m_abs - 10.635))",
    distmod = host.distmod,
    alpha=...
    ...
)

[github-actions]

Before [a5b5410]	After [788080a]	Ratio	Benchmark (Parameter)
4.51±0.05ms	4.81±0.2ms	1.07	benchmarks.TimeSuite.time_evaluate_salt3_passbands
8.35±0.05ms	8.69±0.6ms	1.04	benchmarks.TimeSuite.time_evaluate_salt3_model
698±30μs	716±700μs	1.03	benchmarks.TimeSuite.time_fnu_to_flam
4.43±0.08ms	4.51±0.3ms	1.02	benchmarks.TimeSuite.time_chained_evaluate
1.32±0.05ms	1.33±0.1ms	1.01	benchmarks.TimeSuite.time_apply_passbands
8.84±0.05ms	8.94±0.05ms	1.01	benchmarks.TimeSuite.time_load_passbands
1.35±0.01s	1.35±0.01s	1	benchmarks.TimeSuite.time_make_x1_from_hostmass
16.1±0.3μs	16.2±0.2μs	1	benchmarks.TimeSuite.time_sample_x1_from_hostmass
123±2μs	122±1μs	0.99	benchmarks.TimeSuite.time_sample_x0_from_distmod
28.5±0.4μs	27.9±0.3μs	0.98	benchmarks.TimeSuite.time_make_new_salt3_model

Click here to view all benchmarks.

[mtauraso]

Totally irrelevant to this PR, but as a grad student I've definitely wished for this sort of conversion as a callable library, especially if C stdlib was one of the "basic math" languages that could be converted.

[hombit]

Some general usability issues, most of them are not related to this PR and can be discussed separately:

1. BasicMathNode doesn't have a string representation
2. GraphState doesn't have a string representation and iteration over it fails, e.g. list(BasicMathNode("a", a=1, node_label="node").sample_parameters())
3. BasicMathNode("a", a=1).sample_parameters() cannot be used with [] syntax because node_label is None. However I still could use extract_parameters
4. Math exceptions like 1/0 or log(-1) are hard to track
5. It is not clear how I can a) jit-compile with jax and b) use a random number node.

[jeremykubica]

Thanks @hombit, I filed some issues for some of those:

1. I need to think about how to provide a string representation, because I currently use __str__ for some of the node labeling.
2. Added issue Make GraphState iterable #186
3. Added issue Create a connivence accessor for a single node GraphState #187
4. Agreed. I don't have a great solution here, but I can try adding better debugging (Add better exception handling for math errors #188)
   5a) I don't think jit compiling is possible with this node, so we would need to be careful.
   5b) You can use a random variable as input the same way as within any other node.