Change interpolation method for capacity data

The `interpolate_capacity` function is currently using linear interpolation. However, as pointed out [here](https://github.com/EnergySystemsModellingLab/MUSE_OS/pull/642#discussion_r1944376005), linear interpolation doesn't make much sense because capacity profiles are step-wise. Forward filling (i.e. taking the previous specified value) would make more sense.

We can simply change `interpolate_capacity` to return `data.reindex(year=year, method="ffill")`

I've given this a try, but some of the results changed and I'd like to understand why before implementing this change