add waveform plot to tutorial

Added the following at the bottom of examples/tutorials/visualising_the_results.ipynb:

#%% md

#### Best-Fit Time Domain Waveform plot
Some plots sepcific to compact binary coalescence parameter estimation results can
be created by re-loading the result as a `CBCResult`:

#%%

from bilby.gw.result import CBCResult

cbc_result = CBCResult.from_json("visualising_the_results/example_result.json")
cbc_result.plot_waveform_posterior()
plt.show()

#%% md

Again, notice that the plot is saved as a "waveform.png" in the output dir.

Should I clear output? ie: jupyter nbconvert visualising_the_results.ipynb --to notebook --ClearOutputPreprocessor.enabled=True --output visualising_the_results.ipynb