support non-regular grid in fringe search

[lindyblackburn]

in hops.util.findfringe
currently frequencies are assumed to stack into a regular grid
current DiFX configuration for EHT 2017 breaks this by having discontiguous HOPS channels, with subchannels that do not lie on a regular grid
an FFT fringe search over the entire band needs to be adapted to (approximately) handle this
one option is to place visibilities onto a supersampled regular grid, which is memory intensive but will scale computationally as logN which isn't so bad
can possibly deal with the memory issue by making sure we loop efficiently over time, at each stage we can throw away the "zero-padded" part of the delay spectrum