Fix AGN model and add documentation notebook for it

[hombit]

It is in preparation for the LSST AGN SC Variability meeting next Monday

@jeremykubica I had some problems assigning a node label for the test_create_agn, it complains that the label is duplicated. I guess that I did something wrong in the model...

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Before [e399b4a]	After [402e40c]	Ratio	Benchmark (Parameter)
117±8μs	121±7μs	1.03	benchmarks.TimeSuite.time_evaluate_simple_linear_wavelength_model
30.0±0.4μs	30.9±0.5μs	1.03	benchmarks.TimeSuite.time_make_simple_linear_wavelength_model
4.50±0.08ms	4.58±0.1ms	1.02	benchmarks.TimeSuite.time_evaluate_salt3_passbands
728±20μs	742±40μs	1.02	benchmarks.TimeSuite.time_fnu_to_flam
4.71±0.1ms	4.80±0.09ms	1.02	benchmarks.TimeSuite.time_lightcurve_source
8.67±0.07ms	8.85±0.2ms	1.02	benchmarks.TimeSuite.time_load_passbands
12.6±0.2μs	12.8±0.1μs	1.02	benchmarks.TimeSuite.time_sample_x1_from_hostmass
7.60±0.04ms	7.66±0.1ms	1.01	benchmarks.TimeSuite.time_apply_white_noise
19.4±0.2ms	19.5±0.3ms	1.01	benchmarks.TimeSuite.time_make_and_evaluate_static_sed
48.4±0.7μs	48.9±0.8μs	1.01	benchmarks.TimeSuite.time_make_new_salt3_model

Click here to view all benchmarks.