Implement "Randomize Until Reaches Threshold" Functionality

[WarrenZhu050413]

Currently, Generate Randomizations can only output the lower $\alpha$ percent of randomizations according to some threshold function. Rerandomization methods, however, normally requires that all randomizations go below some threshold value $t$. It would be great if FastRerandomize supported this.

One potential way to implement this is to allow the user to specify num_required and threshold_value, and run the function until num_required number of randomizations that matches threshold_value is sampled. Another paramter max_time could be provided that requires the function to halt after a certain period, or give warnings about the time this rerandomization could take. The function could also calculate based on some preliminary sampling of the time taken.