add original weight name as prefix for mask and threshold weights

[christian-steinmeyer]

If there are multiple prunable weights in a wrapped layer, without this change, the wrapped model cannot be saved to h5 because of duplicate dataset names (the layer has multiple weights called "mask" and "threshold").

There are certainly other ways to solve this issue, but this felt like the most straight forward. Feel free, to improve.

solves #1077

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