[feature] Support setting custom quantile in plot_parameters()

[LeonieFreisinger]

Modify the plot_parameters() function to allow the user to plot a chosen quantile instead of the median.

Therefore, the matplotlib and plotly version will be modified. The quantile will be plotted with absolute value.

[codecov-commenter]

Codecov Report

Merging #853 (54bdd47) into main (f7e09e4) will decrease coverage by 0.02%.
The diff coverage is 91.30%.

@@            Coverage Diff             @@
##             main     #853      +/-   ##
==========================================
- Coverage   87.06%   87.03%   -0.03%     
==========================================
  Files          16       16              
  Lines        4622     4627       +5     
==========================================
+ Hits         4024     4027       +3     
- Misses        598      600       +2     

Impacted Files	Coverage Δ
neuralprophet/forecaster.py	86.13% <60.00%> (-0.13%)	⬇️
neuralprophet/plot_model_parameters.py	62.88% <100.00%> (ø)
neuralprophet/plot_model_parameters_plotly.py	90.64% <100.00%> (ø)

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[ourownstory]

@LeonieFreisinger To simplify things, just change all panels to the specific set quantile (instead of the (default) median).
So, if people call plot_parameters with no args, it plots the parameters of the median.
If they set the quantile, it plots (only) the parameters of the set quantile.

[LeonieFreisinger]

@ourownstory custom quantile for plotting parameters and respective tests are implemented. Would be great, if you could review it.
Thanks!

[ourownstory]

LGTM