Refactor snakefile

[fhg-isi]

Some suggestions:

b) Remove the out commented section convert_fraunhofer from the Snakefile (we have version control, don't we?) or comment its meaning.

c) Use snake_case naming conventions for rule names => convert_ewg instead of convert_EWG?

d) rename rule and script compile_cost_assumptions to main to clarify what part is the main script / entry point.

e) Clarify if project is

1. only about cost or if
2. it also contains other data.

If 1. maybe rename project to technology-cost If 2. maybe rename output files to outputs/technology_{year}.csv