Add gumbel-softmax for classification

# Feature
## Desired Behavior / Functionality
An alternative to the currently utilized approach of building a linear combination of materials may be the use of a gumbel-softmax as described here https://arxiv.org/abs/1611.01144 .

## What Needs to Be Done
- Add gumbel softmax in network -> eps conversion in https://github.com/esa/NIDN/blob/be21eeb08b01782ee6c2735f193796c2b4bcbd2c/nidn/training/model/model_to_eps_grid.py#L199
- Add a cfg flag to select

## How Can It Be Tested
Run one of the notebooks using the classification with it.