filter: updates kernel::fft_filter_xxx documentation to better explain how it works and how to use it in other blocks.

Addresses concern raised in issue #662.
cmake: better setup for finding QWT; otherwise, it can be forced to incorrectly "find" non-existent QWT libs.
uhd: Fixed "reducing chan number" bug

In some cases, there could be an error when reducing the number
of chans for UHD device in GRC (e.g. going from 2 chans to 1).
This adds another check which will alert the user.
filter: allow small rounding error in oversample rate setting for pfb_channelizer.
filter: fixes tag propagation through the pfb_channelizer.