fix: double-extracting congestion lower, not extracting upper

[ghost]

Fixing a typo, see #23 for a description of the problem, and https://github.com/MATPOWER/matpower/blob/master/lib/idx_brch.m for more info on what we are extracting from the matpower solution.

This is needed for congestion and solar curtailment analysis.

[rouille]

Did you test the change on a MATLAB output file?

[ghost]

Yes, the change extracts a matrix of the same dimension but with different values (as expected).

[rouille]

After the merge, I will need to delete the current outputs and re-extract the data. @kasparm, can you look at this PR when you have a sec. I believe @danielolsenIV needs the new file for data analysis. Thanks

[ghost]

Deleting and re-analyzing is not urgent, I've managed to do the extraction locally and I am doing analysis based on these files. I could also scp the updated pickle files back to the server if that's easier than deleting and re-analyzing.

[rouille]

You downloaded all 61 MATLAB files from the server and did the extraction on your laptop? It is not very complicated to re-extract the files. I will do it once the PR is approved and the changes are checkout on the server. No worries.

[ghost]

I did exactly that. I am very curious to solve this problem and didn't want to wait any longer than I had to!

I also made one more small fix, I had improperly specified which one was the 'upper' limit dual var and which one was the 'lower' limit dual var (the MATPOWER documentation is quite vague), this is fixed now.