Allow convolution of resolution with SQw

Currently, when applying instrument resolution to a SQw object, the Fourier transform of the resolution function is applied multiplicatively to FQt, then Fourier transformed to SQw. This works for functions with convenient Fourier transform properties, but for some functions it may be easier to convolve the function itself directly with SQw instead.

This issue is the part of #24 which has not been fixed by PR #719. It may be easier to do alongside #269, since much of that area of code will be refactored anyway.