Refactor fuel-chain emissions plugin

Dramatically reduced "pollution" of Qernel::Graph by the FCE module (no
pun intended) by extracting functionality into a new FCECalculator
class, which may be accessed as Graph#fce.

Expects quintel/etsource@98c01a9 or newer, which has the FCE load
profiles moved into the dataset directory such that each country can
have it's own.

Closes quintel/etsource#708
Default test cache store is :file_store, wtf?

Data was being carried over from one test run to another, leaving me
wondering why a change to a Gquery fixture was running an old query time
after time after time after...

Apparently this is the Rails default. I don't even...