Renamed plot_building_counts.py

While reviewing https://gitext.gfz-potsdam.de/dynamicexposure/globaldynamicexposure/loss-calculator/-/merge_requests/8 a conversation about naming conventions in python came up. For this, it is good to look at pep8.

Modules should have short, all-lowercase names. Underscores can be used in the module name if it improves readability. Python packages should also have short, all-lowercase names, although the use of underscores is discouraged.

Thus, I suggest renaming this python script from camel-case to underscored, all lower-case.

/cc @tara