add support for skip value or skip function

[DanielJDufour]

this would be different from nodata, which is tracked.

skipped would be completely ignored

[DanielJDufour]

errr.. is that the same as filtering? https://github.com/danieljdufour/calc-stats#filtering

should filtering be treated as a noData function, such that filtered out values are seen as invalid?

[DanielJDufour]

this is currently the case, so it seems a separate skip function is not needed

[DanielJDufour]

but might be more ergonomic to provide an array of skip values versus having to define a filter function, would also enable cross process communication