Add regional FCE data, refactor FCE plugin

The Qernel::Graph FCE plugin has been rewritten to extract most of the code to a separate class: `FCECalculator`. This class, which is responsible for calculating the CO2 attributes of FCE-enabled carriers, can be accessed as `Graph#fce`.

Both FCECalculator, and the two GQL functions – `UPDATE_FCE` and `FCE_START_VALUE` – now use shares (0.0-1.0) instead of percentages (0.0-100.0), and ETSource has been updated to reflect that.

This needs to be merged with:

* quintel/etsource#763
* quintel/atlas#40