Support Scikit Learn Pipeline

[danuker]

Since SymReg works best with normalized data, it would be ideal to allow it to be used in a sklearn pipeline, with a StandardScaler(with_mean=False) preprocessor.

Make sure this is possible, and add an example to the README.

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Alternatively (if Pipeline support needs sklearn dependency): have an argument to just normalize the data inside. Divide by the median of the data for the regression, then multiply back when returning the resulting functions.