Add "artificial series" of basic statistics functions

[breznak]

From a user feedback, it should be possible to add (simple) statistical functions as a new series, for example: mean on window 1000 for series 'consumption' this series than can be used for highlighting of interesting sections.

We should probably look for a JS based statistical "frameworks"/functions.

Implementation:

• functionality to add (remove) artificial series to the graph (UI)
• modules to compute statistics, outliners, ...

Related: #46 #47 #21 #31