create both sample_weight and freq_weight for fit

[heimengqi]

• Make StatsmodelsLinearRegresssion allow fractional weight for each individual sample, which is sample_weight. Rename the weights used as the count of observations for corresponding sample_var as freq_weight.
• For all the child classes inherit from _OrthoLearner, only expose freq_weight and sample_weight when the final stage is StatsmodelsLinearRegression or its parent class.
• Fix a few places to make sure it works consistently with un-summarized data when both weights exist.

[kbattocchi]

Looks good, I've mostly just suggested some minor tweaks to some of the docstrings