Filter

[Oxid15]

This PR adds Filter and IteratorFilter datasets into cascade.data.
Filters accept functions that return bool values when receiving dataset items and then use their output to filter values.

For regular datasets with __len__ defined the Filter is Sampler meaning that it changes the length of a dataset
For datasets that are built around iterables Filter is Modifier

If Filter makes the length of a regular dataset 0 then Sampler will raise an AssertionError on creation