Rethink SP local inhibition

[Oblynx]

The SP tiebreaker is necessary to enforce strict sparsity control, but it has undesirable sideffects in other properties:

• sp(z) becomes stochastic (random seed from hash(z)?)
• noise robustness suffers and doesn't improve a lot with training

This points to a need to rethink inhibition, and not do it by comparing against a threshold, but more low-level, by ordering indices and picking the highest activations.

[Oblynx]

Implemented in this commit, but only for global inhibition.