fix: correct bug with carried-over gencosts for new plants

[danielolsen]

Purpose

Fix the bug, where new renewable generators added after a thermal generator inherit the thermal generator's cost curve coefficients.

What the code is doing

The first commit reproduces the code in a test, the second commit fixes it by ensuring that no values are carried over.

Time estimate

1 minute.

[BainanXia]

Good catch!

[rouille]

Should we move in the for loop the other too (new_branch, new_plant, etc.)? I mean we need to set the value of the columns to 0 for all of them in principle.

[danielolsen]

Should we move in the for loop the other too (new_branch, new_plant, etc.)? I mean we need to set the value of the columns to 0 for all of them in principle.

The problem that is specific to gencost is the if block which updates those entries only sometimes. It should not be a problem for the other loops which do not have this feature, but it is probably good practice to move them all, to avoid this issue potentially arising later.

[danielolsen]

The analogous line has been moved inside the loops in the other add methods.

[rouille]

Makes sense