Fix/multiple in out transformer

[NargesChinichian]

fix #802, fix #799, fix #541, fix #599, fix #507

fix transformer_constant_efficiency_fix to accept multiple inputs OR multiple outputs.

Changes proposed in this pull request:

• transformer_constant_efficiency_fix and transformer_constant_efficiency_optimize accepts multiple in/outputs.

transformer_constant_efficiency_fix

• For multiple inputs, "DISPATCH_PRICE"s (variable_costs) and EFFICIENCY (conversion_factors parameter of Transformer object) are assigned to input flows and "INSTALLED_CAP"s (nominal_values) are assigned to output.
• For multiple outputs, same as above but DISPATCH_PRICE and EFFICIENCY are provided to output flows and input flow is left empty
• if the values of the parameters DISPATCH_PRICE, EFFICIENCY are not provided as a list, there is an error raised
• the case multiple input and multiple output (like a CHP) hasn't been covered yet

transformer_constant_efficiency_optimize

• For multiple inputs, DISPATCH_PRICEs (variable_costs) and EFFICIENCY (conversion_factors parameter of Transformer object) are assigned to input flows and INSTALLED_CAP (existing parameter of Investment object), MAXIMUM_ADD_CAP (maximum parameter of Investment object) and SIMULATION_ANNUITY (ep_costs parameter of Investment object) are assigned to output.
• For multiple outputs, same as above but DISPATCH_PRICE and EFFICIENCY are provided to output flows and input flow is left empty
• if the values of the parameters DISPATCH_PRICE, EFFICIENCY are not provided as a list, there is an error raised
• the case multiple input and multiple output (like a CHP) hasn't been covered yet

The following steps were realized, as well (if applies):

• [x] Use in-line comments to explain your code
• [x] Write docstrings to your code (example docstring)
• [x] For new functionalities: Explain in readthedocs
• [x] Write test(s) for your new patch of code (pytests, assertion debug messages)
• [x] Update the CHANGELOG.md
• [x] Apply black (black . --exclude docs/)
• [x] Check if benchmark tests pass locally (EXECUTE_TESTS_ON=master pytest)

_{For more information on how to contribute check the CONTRIBUTING.md.}

[Bachibouzouk]

Multiple outputs and one input

mvs_tool -i tests/benchmark_test_inputs/Feature_output_flows_as_list -ext csv -f

t1 = solph.Transformer(
  label='diesel_generator',
  inputs={Electricity (diesel): solph.Flow(max=1, min=0)},
  outputs={Electricity: solph.Flow(variable_costs=0.3, max=1, nominal_value=1250, min=0), Heat: solph.Flow(variable_costs=0.5, max=1, nominal_value=1250, min=0)},
  conversion_factors={Electricity: [0.3], Heat: [0.5], Electricity (diesel): [1]},
)

Multiple inputs and one output

t1 = solph.Transformer(
  label='diesel_generator',
  inputs={Electricity (diesel): solph.Flow(min=0, max=1), Electricity: solph.Flow(min=0, variable_costs=0.15, max=1)},
  outputs={Electricity: solph.Flow(nominal_value=1250, min=0, max=1)},
  conversion_factors={Electricity (diesel): [0.6], Electricity: [1]},
)

[Bachibouzouk]

I only changed the changelog and the tests did not trigger, I will merge as the previous status was passing