Fix PyPSA exporter

[rouille]

Pull Request doc

Purpose

Add missing rename value when converting a Grid object to a Network object. I forgot to include it when I updated the export_to_pypsa function from changes made in #608. As a result, the c1 column will not be renamed to marginal cost as expected by PyPSA

What the code is doing

N/A

Testing

I found the bug in the ben/import branch when trying to convert back to a Grid object a Network object previously created from a Grid object.

Where to look

You can compare what is done in #608 (see here)

Usage Example/Visuals

N/A

Time estimate

2min

[BainanXia]

Make sense. I remember we had the discussion that we would linearize the quadratic cost curve to calculate the marginal cost instead of taking c1 directly and we should rename it accordingly.