[a.fixBA_plot] Remove global variables from ba_plot (912)

Merging branch 'a.fixBA_plot'  into 'main'.

* The implicit parsing of commandline arguments is eliminated from the `ba_plot` module;
therefore, importing `ba_plot` will have no side-effects.
Originally, the implicit parsing of commandline arguments during importing led to crashes when
the module was imported in a different environment, like a Jupyter notebook.

* All global variables are removed from the `ba_plot.py` module and plot arguments are made explicit to avoid unexpected problems
due to corruption of global variables. In the originall scheme, storing the plot arguments as global variable would lead to strange
behaviour if two examples were run sequentially, since the global plot arguments would persist in the global scope from one example
to the other.

Related to #912

See merge request !2525