Add two features requested in the review of PR185

[jeremykubica]

Adds two new features that were requested in the review of #185

1. Adds a convenience function that allows the user to access a node's variables directly when there is only a single node in the GraphState (see Create a connivence accessor for a single node GraphState #187)
2. Allows iteration over each of the states within a GraphState that has multiple samples (see Make GraphState iterable #186)

Closes #187
Closes #186

[github-actions]

Before [9ea64af]	After [0b123b6]	Ratio	Benchmark (Parameter)
765±30μs	786±500μs	1.03	benchmarks.TimeSuite.time_fnu_to_flam
4.78±0.09ms	4.78±0.2ms	1	benchmarks.TimeSuite.time_evaluate_salt3_passbands
28.3±0.4μs	28.4±0.4μs	1	benchmarks.TimeSuite.time_make_new_salt3_model
1.36±0.01s	1.35±0.01s	1	benchmarks.TimeSuite.time_make_x1_from_hostmass
124±2μs	123±2μs	1	benchmarks.TimeSuite.time_sample_x0_from_distmod
1.42±0.06ms	1.41±0.03ms	0.99	benchmarks.TimeSuite.time_apply_passbands
8.72±0.7ms	8.64±0.9ms	0.99	benchmarks.TimeSuite.time_evaluate_salt3_model
9.07±0.07ms	8.97±0.1ms	0.99	benchmarks.TimeSuite.time_load_passbands
15.8±0.2μs	15.7±0.2μs	0.99	benchmarks.TimeSuite.time_sample_x1_from_hostmass
4.54±0.1ms	4.39±0.3ms	0.97	benchmarks.TimeSuite.time_chained_evaluate

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