Unexpected output th.stats.std

[Yejashi]

There is an issue with the th.stats.std function where single profiled thicket results in Nan's value due to pandas use of sample standard deviation instead of population standard deviation.

A way to get around this issue is to use lambda x: np.std(x) in the aggregate function of th.stats.std.

i.e df = thicket.dataframe[columns].reset_index().groupby("node").agg(lambda x: np.std(x))