Add `inplace=False` argument to `FeatureSet.filter()`

[jbiggsets]

It may be useful in some cases to filter the featureset and return a new filtered FeatureSet object, rather than filtering in place. For example, if the user has an existing FeatureSet and wants to do some analysis on a subset of the features, while leaving the original object alone.

In addition, this could be useful in addressing #454. If users want to perform filtering only on a subset of features, it would be convenient to filter the FeatureSet into separate objects, perform transformations on the features in one of these FeaturesSets, and then combine them.

[stale[bot]]

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