GetCoefficientStatistics in Microsoft.ML.Learners.LinearModelStatistics  needs to be public

[GetCoefficientStatistics](https://github.com/dotnet/machinelearning/blob/master/src/Microsoft.ML.StandardLearners/Standard/ModelStatistics.cs#L327) in Microsoft.ML.Learners.LinearModelStatistics  needs to be publicly accessible. 

The statistics calculated provide no value, if they can't be accessed. 