Fine-tune selection of cities

[ruestefa]

Currently (v0.13.11), the city labels are selected as follows:

• All captital cities.
• All additional cities of a certain size, which depends on whether the domain is considered continental-, regional- or country-scale.

This works OK for Europe. However, the size thresholds are not appropriate for other regions like SE Asia with many huge cities or Siberia with barely any cities. Also, sometimes two cities in close vicinity of similar will both be shown when one would (visually) suffice.

TODO: Fine-tune the selection of cities.

Possibilities:

• Define the size thresholds based not only on the scale, but also on the region as identified by the release point.
• Hardcode some cities, either including or exluding them.

[ruestefa]

Added list excluded_cities to pyflexplot.plotting.map_axes::MapAxes::_ax_add_cities::is_of_interest to exclude the city Incheon. Add additional cities if necessary.

[ruestefa]

Note: Instead of hardcoding the cities, the list of excluded cities would better be added to MapAxesConfig and defined in pyflexplot.plots, where all the other stuff is defined. This would also make it easy to exclude cities based on the model, domain etc.