Add a helper copy method to GraphState

[jeremykubica]

This lets us eventually copy and modify GraphStates to try other configurations (e.g. setting t0 to 0.0).

[github-actions]

Before [e04f951]	After [e9030b3]	Ratio	Benchmark (Parameter)
7.51±0.02ms	7.53±0.04ms	1	benchmarks.TimeSuite.time_apply_white_noise
19.0±0.2ms	19.1±0.2ms	1	benchmarks.TimeSuite.time_chained_evaluate_sed
648±20μs	650±20μs	1	benchmarks.TimeSuite.time_fnu_to_flam
4.59±0.08ms	4.59±0.08ms	1	benchmarks.TimeSuite.time_lightcurve_source
87.8±2μs	87.7±1μs	1	benchmarks.TimeSuite.time_make_and_evaluate_step_model
12.3±0.2μs	12.3±0.2μs	1	benchmarks.TimeSuite.time_sample_x1_from_hostmass
32.7±0.7ms	32.4±0.4ms	0.99	benchmarks.TimeSuite.time_additive_multi_model_source
536±8μs	530±10μs	0.99	benchmarks.TimeSuite.time_apply_passbands
4.41±0.07ms	4.38±0.07ms	0.99	benchmarks.TimeSuite.time_evaluate_salt3_passbands
114±6μs	113±7μs	0.99	benchmarks.TimeSuite.time_evaluate_simple_linear_wavelength_model

Click here to view all benchmarks.