Avoid doing unnecessary FFTs

In many situations the visibilities do not cover the full u,v space being Fourier transformed. Whenever this is the case, ther is no need to carry out the full FFT over the uv grid; a part of the FFT in one direction can be omitted. If the omissions are done in the direction that is more expensive to transform (in our case the u-direction), the time savings can be quite substantial.