Special values filters and selectors

[koalive]

Since issue #9 transformation functions are expected to check if the data contains inf / na / missing values, and the MissingFilter allows to remove entries containing missing values (for a given feature). It would be convenient to include similar filters for inf and na values, and selectors to remove columns containing special values.

[koalive]

Example for a Selector discarding features with NaN values: Selector(x -> !any(isnan.(x)))