Update voxel size for "mitochondria2" model

[constantinpape]

I have copied the voxel size from the prev. model, but it should be determined based on the training data.
See https://github.com/computational-cell-analytics/synapse-net/blob/main/synapse_net/inference/inference.py#L116.

[lufre1]

When averaging the voxel sizes of both training dataset we get approx 1.45nm. I'll update the code accordingly.