Infer default class key from yaml file containing road user classes

[martinbaerwolff]

OTGroundTruther/OTGroundTruther/model/model.py

Line 215 in 9134284

if DEFAULT_CLASS_KEY is None:

We use a default class key for specifying, whether the road user class has to be specified in the gui or not. If only one class is possible, this is not needed. Hence, we could just infer the number of classes from the yaml file containing the road user classes and if that number is one, we could set that class as the default. This way, nobody would have to change the code to set the default class