Fix printing of file name in template cpp files

Maybe we should use https://jinja.palletsprojects.com/en/2.11.x/api/#jinja2.StrictUndefined when creating the jinja2.Enviroment to generate errors on undefined values.

Would have catched this error.

>   #include "{{class_name}}.h"
E   jinja2.exceptions.UndefinedError: 'className' is undefined

pystencils_walberla/templates/Sweep.tmpl.cpp:25: UndefinedError

I'll make a PR for this.

merged

mentioned in commit c234592a

mentioned in merge request !7 (merged)