Frequency-dependent drag

Utilizing two streaming filters, the barotropic velocity is band-pass filtered at the M2 and K1 frequencies, and the frequency-dependent internal wave drag is applied to the M2 and K1 velocities separately. Limitations of this release:

1. The filters do not have restart capability,
2. The frequency-dependent drag is implemented for scalar form of the drag and is not yet compatible with directional or tensor form of the drag.