Implement OffsetTransformer as another generator model

[smartie2076]

PR #90 must be merged first, in case that this branch is merged at all

This introduces the offsetTransformer from oemof solph, which required oemof to be updated to oemof-solph v4.0

Homer might be an interesting comparison: Generator model Homer

This is placed on hold due to more urgent issues.

Changes applied in this pull request:

• [x] Reorder variables in constants.py
• [x] Add new parameter GENSET_WITH_EFFICIENCY_CURVE to inputs
• [x] Create B.process_generator_settings to evaluate different generator settings
• [x] Refactor genset functions in G2a and G1 to make them explicit
• [x] Implement OffsetTransformer as G2a.genset_fix_capacity_efficiency_curve_and_minimal_loading. Note: Efficiency of the new OffSet Transformer with efficiency curve uses the GENSET_EFFICIENCY as maximum efficiency, and GENSET_EFFICIENCY/2 as minimal part-load efficiency
• [x] Add benchmark test for generator with efficiency curve (OffsetTransformer), only termination-check
• [x] Retrieve CONSUMPTION_FUEL_TIMESERIES_KWH from simulation results and store to csv
• [x] Calculate generator efficiency (EFFICIENCY_GENSET_TIMESERIES), store to csv
• [x] Generate and store a graph of the efficiency curve if GENSET_WITH_EFFICIENCY_CURVE==True

[smartie2076]

CONSUMPTION_FUEL_TIMESERIES_KWH and EFFICIENCY_GENSET_TIMESERIES in csv output file

(Set `save_to_csv_flows_electricity_mg == True)

Example generator efficiency curve with OffsetTransformer / GENSET_EFFICIENCY_CURVE==True:

(Set save_to_png_flows_electricity_mg==True) (Note: For GENSET_EFFICIENCY_CURVE==False this plot would be a straight line, so it is not displayed.)

[smartie2076]

@tobirieper this is the PR that includes the new OffsetGenerator feature.

[smartie2076]

It is not possible to activate a generator with efficiency curve which does not have minimal loading. The simulation runs though, but the efficiency curve is not applied. This needs to be checked, a test written, and fixed.

[smartie2076]

Fixed the fuel price issue and rebased. @tobirieper this now also features the operational hours of the diesel generator.