random selection of learning cells from bursting columns

[floybix]

In bursting columns, the winner / learning cell is selected by maximum total excitation (including a penalty on cells with inactive segments, to encourage efficient use of all cells).

Currently ties are broken by keeping the first. The problem with this is, a set of blank columns bursting in

• context A will each end up with cell 0 learning,
• context B will each end up with cell 1 learning,
• context C will each end up with cell 2 learning, etc.

So after depth contexts, the next one will be fully identical to context A.

Instead we need to break ties randomly, unlocking the combinatorial capacity for representation.