masked_select: Filtering tensors with a boolean mask tensor
given a 2d array/tensor, how would i do the arraymancer equivalent of this numpy expression:
```python
a[:,a.mean(axis=0) > 0.5] = -1
```

I think it's probably some combination of the `map/apply/fold_inline`, but it's not obvious how to do that.
Once I understand, I can open a PR with some common examples like this if it's helpful.

