Plot Refactoring

[pabloitu]

closes #260
closes #258

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Fixes issue #(please fill in or delete if not needed).

Type of change:

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• Bug fix (non-breaking change which fixes an issue)
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• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
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• I have added tests that prove my fix is effective or that my feature works
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[pabloitu]

rasterio is breaking the macos python 3.9 CI build for some reason. Need to fix

[pabloitu]

The refactoring is ready. All the tasks done for this refactoring are explicit in #260. Would be great if @mherrmann3, @bayonato89 and/or @Serra314 can also take a look. The main changes are:

1. Clearly defined main and helper plotting functions, as well as removed unused functions (need feedback that im not removing something used)
2. Harmonized function signatures (e.g., args/opt_args/kwargs):
   2.1 All the plot functions now have identically named arguments
   2.2 Removed the plot_args dictionary as argument, in favor of using direct keyword arguments (to be consistent with matplotlib and seaborn).
   2.3. Cleaned all function signatures for only those arguments that are essential for a function. Fine-tuning args (e.g. title_fontsize, legend_title) are now part of optional kwargs, and described in the docstrings.
   2.4. The plots.py module has a DEFAULT_PLOT_ARGS dictionary (similarly to matplotlib.rcParams), that contains the default values for all plotting. Any arg/kwarg passed to a function, replaced the defaults. I opted by this simple solution, but if required, we can refactor in the future to a PlotConfig class, in the plotly style.
3. Added type hinting (e.g., plot_catalog(catalog: CSEPCatalog) ) for all the functions, so there is awareness of the argument type that a function requires.
4. Refactored all plot_{catalogforecast_test} (e.g., number_test, spatial_test, likelihood_test) onto one function (plot_distribution_test, please advice for a more precise name), since the logic was identical. It now calls a helper function that automatically retrieves the annotations and labels for each test (e.g., spatial_test, number_test).
5. Revamped plot_cumulative_number_versus_time and plot_magnitude_versus_time: They now use datetime for the x label, and the latter sizes the events by their magnitude (for the good looks)
6. Added unit tests for all the plotting functions.
7. Basemaps now caches downloaded data, so it becomes 1000% quicker to plot multiple catalogs/forecasts. Also a geotiff file can be used as basemap, as long as it is in the same projection required.
8. The test_plots.py module, has a flag show_plots. If set to True, you can test the plots and see all the possible plots (with simple data) that pycsep can make.

This PR should go to a v1.0.0 candidate branch, since its not backwards compatible.