Parametrizing MixedDataLoader option for discrete variable

[timonmerk]

I have a question regarding the MixedDataLoader

The current docstring specifies that:

Sampling can be configured in different modes:

1. Positive pairs always share their discrete variable.
2. Positive pairs are drawn only based on their conditional,
   not discrete variable.

I am not sure how it's currently possible to do that?

Maybe an option might is to add a boolean keyword positive_sampling to be either conditional or discrete_variable?

In addition, as already mentioned in the docstring of the get_indices function, the priorof the discrete distribution could be specified https://github.com/AdaptiveMotorControlLab/CEBRA/blob/0378db0b2431d0c50a1e9b80aa1b865869586851/cebra/data/single_session.py#L305

I will reference a PR here also