WIP: Adding notebook with example of using a "black box" likelihood (#3169)

* Adding notebook with example of using a "black box" likelihood

* blackbox_external_likelihood: add checks of the gradient function

* blackbox_external_likelihood: updated notebook to add profiling

Author: mattpitkin

docs/source/examples.rst

@@ -19,6 +19,7 @@ Howto
    notebooks/LKJ.ipynb
    notebooks/live_sample_plots.ipynb
    notebooks/sampling_compound_step.ipynb
+   notebooks/blackbox_external_likelihood.ipynb
 
 Applied
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