Feature/improve tracer example

This merge request contains a few more changes made to the 1ptracer example. This depends on !1914 (merged) and !1915 (merged).

TODO:

• put online links to sub-sections in doc/problems.md -> edit: relative links seem to work fine
• Put caption and introductory text in doc/1pintro.md
• Put caption and introductory text in doc/tracerintro.md
• Clean up and de-clutter properties_1p.hh
• Clean up and de-clutter problem_1p.hh
• Clean up and de-clutter spatialparams_1p.hh
• Clean up and de-clutter properties_tracer.hh
• Clean up and de-clutter problem_tracer.hh
• Clean up and de-clutter spatialparams_tracer.hh

All of these points target at making the resulting docus in doc/1pmodel.md and doc/tracermodel.md nicer and more readable.