Filters/extra filters (#206)

* Added `deduplicate_adjacents`. Solves #201

* also pass arrays and meta to function based filters (e.g map_y). Fixes #195

* fixed deduplicate_adjacent, added load_var

* Adds the word "smoothing" to the filters' readme to ease docs search

* implemented trendlines

* Trend legend is "Trend"

Author: dbuezas