bug: MappingSaturation vor roads-cars returns only linear models

[Gigaszi]

e.g. https://ohsome.org/apps/dashboard-test/de/#backend=oqtApi&topic=roads&adminids=-62403&indicators=mapping-saturation

https://ohsome.org/apps/dashboard-test/de/#backend=oqtApi&topic=roads&adminids=-285864&indicators=mapping-saturation

Skipping model "Nls Michaelis-Menten Model" due to RuntimeError: Conversion rules for rpy2.robjects appear to be missing. Those
rules are in a Python contextvars.ContextVar. This could be caused
by multithreading code not passing context to the thread.