recombination_freq_threshold implementation

[awohns]

On way of regularizing our results is to impose a "recombination frequency threshold", implemented as a parameter called recombination_freq_threshold in ldgm.brick_ts().

This function currently bifurcates edges into two bricks if the frequency of an edge to the right of a recombination event is greater than the given threshold. So, an edge that is high frequency to the left on the recombination and low frequency to the right will not be bifurcated.

In the future, a better implementation is to say that the frequency of a brick is the union of its unique descendants anywhere it exists, and compare the threshold to that value.

Our approach is internally consistent however, since we use the frequency of an edge the first time it appears as the frequency of the brick.