Use sampling in select_univariate.

[k15z]

Currently, when applying GaussianCopula to large datasets, the univariate selection procedure takes up a lot of the computation time. We should sample from the dataset when trying to find the best fit univariate; if necessary, we could do some math and figure out the appropriate number of rows for X confidence level with Y hypothesis, but I think ~100 rows should be more than enough.