Fine-tune positioning of city labels

[ruestefa]

Right now (v0.13.11), city labels are all positioned the same relative to the city marker. In some cases, this leads to overlap with borders, rivers or other city labels.

TODO: Fine-tune city label positioning to prevent the most egregious cases of overlap.

Possibilities:

• Given this is a common problem, maybe there is already some information on label positioning available in the shapefiles. (Probably not, but this should be checked at least.)
• Hard-code fine-tuned offsets for some cities in the code.

[ruestefa]

This could relatively easily be implemented in pyflexplot.plotting.map_axes::MapAxes::_ax_add_cities by hardcoding lat/lon offsets for certain cities, analogous to the hardcoded exclusion of certain cities (see #5 ). Although, it would be cleaner to add a dictionary of offsets per city to MapAxesConfig and define it in pyflexplot.plots, which would also make it easy to make the offsets dependent on the model, domain etc.