Show number of year-on-year slopes in degradation summary plot

[kandersolar]

Is your feature request related to a problem? Please describe.
Sometimes it is handy to know how many year-on-year slopes made it into the final histogram, for example when evaluating different data filtering strategies.

Describe the solution you'd like
Show the number of year-on-year slopes on the summary plot next to the Rd and CI values, something like this:

Describe alternatives you've considered
It is possible to DIY after calling degradation_summary_plots, courtesy of @mdeceglie:

def get_annotations(ax):
    annotations = [child for child in ax.get_children() if isinstance(child, matplotlib.text.Annotation)]
    return annotations

n_values = ta.results['sensor']['yoy_degradation']['calc_info']['YoY_values'].notnull().sum()
fig = ta.plot_degradation_summary('sensor')
note = get_annotations(fig.axes[1])[0]
newtext = note.get_text() + f'\n# values = {n_values}'
note.set_text(newtext)
note.set_position((0.55, 0.6))

Additional context
Folks thought it was a good idea to only add this to the annotation when detailed=True.