vectorization of the interpolation in PyGSM2016

The 2008 maps have vectorized interpolation (i.e. there's no loop through each frequency in the generate() function). However, the 2016 generate() method does this loop. I feel like this is unnecessarily inefficient, but wasn't game to fix it up myself without a better knowledge of exactly what's going on in the loop.

Is there a reason it has been done differently? If not, would you like me to have a go at vectorizing it?