nominal_value=1 bounds flow variable without min/max/fix being set

[lensum]

Describe the bug If the attribute nominal_value=1 is set in a flow without setting either one of min/max/fix, the flow variable seems to be bounded to 1.

To Reproduce Steps to reproduce the behavior:

1. Set up an energysystem model
2. Create a sink with fixed demand > 1, fed from a transformer
3. Set nominal_value=1 in the transformer outflow
4. Try to solve the resulting model

Expected behavior

• A raised warning if nominal_value is set without min/max/fix being set
• Documentation of default values for min/max arguments

Desktop (please complete the following information):

• oemof.solph version: 0.4.5

Additional context Might be related to: https://github.com/oemof/oemof-solph/issues/818

[jnnr]

This is because the default value of max is 1. https://github.com/oemof/oemof-solph/blob/7e341fdd041c38da8dfbb95867bb3a5bbc9bbe15/src/oemof/solph/network/flow.py#L182-L183 We could tell about this in the docstring.

[p-snft]

We should just state that ’max’ has a default value of 1. When we touch the docstring, we should also delete the parameter fixed, which is no longer used.

[lensum]

@joroeder @DFuh this might be fixed with #881 ?