Separate updatevars! for physical space and Fourier space

[navidcy]

Updating variables in physical space requires an inverse Fourier transform, which in turns requires a deepcopy(). Sometimes, though, we only need to update the Fourier transform of variables. Therefore if we split updatevars! into two separate function we might gain on allocation.

[navidcy]

@glwagner, how about something like 73bff09?

[navidcy]

At the moment no module will benefit from this so I'm closing.