Change interpolation method for capacity data

[tsmbland]

The interpolate_capacity function is currently using linear interpolation. However, as pointed out here, 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