Allow incremental transmission expansion in the myopic optimisation

[martacki]

Working implementation exists here (only tailored to a slightly different workflow). Savely ignore changes in the NTC script(s). Still, might be useful:
open-energy-transition#41

Describe the feature you'd like to see

If set to vopt, copt or alike, the myopic optimisation can already massively expand the grid during the first planning_horizon. As this is often not feasible, it would be better to allow for incremental increases in the base_year, and then allow for further increases in the following planning horizons.

Conversely, if initially set to v1.05 or c1.05, the model might hit the upper limit already during the first few iterations and won't have further expansion options.

I am attaching a working implementation, which might need a bit of tailoring for pypsa-eur upstream.

I also encountered an issue with this line:
lines_s_nom = n.lines.s_nom.where(n.lines.type == "", _lines_s_nom)
where it was setting the s_nom to a horrific high value, while a line type was present. I didn't have proper time to debug this, so maybe it was also just an issue on my side.