Stand_by_cost is not implemented correctly

[barazesh]

Checklist

• I am using the current master branch or the latest release. Please indicate:

Describe the Bug

The standby cost for generators and links is not implemented correctly and subsequently its not present in the the objective function.

The issue is caused by the way that the feature is implemented. While all the other parts of the objective function are appended together in a list and then merged together, the standby cost is added to the objective function directly. The problem is that this is done before the other parts of the objective function are merged and therefore its gets overwritten

Possible solutions

As far as I see, this issue can be solved in two ways:
-either the method of adding the standby cost to the objective function is harmonized with the rest of the code in

PyPSA/pypsa/optimization/optimize.py

Line 161 in f94aeee

m.objective = m.objective + (status * stand_by_cost).sum()

-or if there is a specific reason for this feature to be written in this manner, it can just be moved down after the construction of the objective function in

PyPSA/pypsa/optimization/optimize.py

Line 199 in f94aeee

m.objective = sum(objective) if is_quadratic else merge(objective)