Simplified cokes production to make it demand-driven

The cokes production was modelled in a way that it produced constant amounts of cokes, even if the steel production using blast furnaces was 0% in the scenario. In case of extreme scenarios, this resulted in unexpected coal demand.

The following changes are made to make the cokes production demand-driven:

- the import and export converters are removed from the cokes distribution converter (https://github.com/quintel/etsource/commit/bd5112bd0aa9e9affe46e4e7138943810e194948)
- various queries are removed (e.g. that defined demand of the coal_gas distribution and the cokes oven converters) (https://github.com/quintel/etsource/commit/cffb43445c73b748a302ffad2318e33750dc607b)
- various links are defined as `reversed shares` to enable correct calculation of the graph (https://github.com/quintel/etsource/commit/58afee1e65126f609fd3d68caab41a1f38790706)
- an attempt is made to switch the direction of the coupling carrier (https://github.com/quintel/etsource/commit/5e543b79dc4c2af3119715cdfa54942825d3cb00, https://github.com/quintel/etsource/commit/ddc7a86feb3d5e74fec0da28590a3cf875ef462a, https://github.com/quintel/etsource/commit/a7ae44368d4df867bc466526e47640bc9a613b1e) but in the end, this did not work out
- the cokes consumption and cokes transformation converters are therefore merged to avoid issues with the desired the direction of the coupling_carrier (from right-to-left instead of left-to-right)
(https://github.com/quintel/etsource/commit/1a33a7cdebb54ed0a9923c659b88abbb133c6288)
- new efficiency files are added for the merged `energy_cokesoven_coal` converter (https://github.com/quintel/etsource/commit/e3fdb36c35415c8e4cb6f15ef5882c099ba16aad)

![cokes](https://f.cloud.github.com/assets/4319656/2184698/82d4873e-97c5-11e3-9fe0-e150bcd236a1.jpg)