Expose filter method

Alternative to filterOnKeys and filterOnValues
Replace apply with applyValues

The apply method wraps the calling of map on the underlying RDD, and
reconstructing a Data object. The use of apply and applyValues is more
semantically matched to the Spark operations map and mapValues.
Add a crop method

- Supplements the previous Images.plane() method, which is now a
special case of Images.crop()
- Adopts numpy slicing notations
Move detrend and normalize to TimeSeries

- Both implicitly or explicitly assume time series data
- More appropriate to have them as TimeSeries methods
Add a convolve method on TimeSeries
Add useful repr for data objects