Small float internals changes

Hi Scott,

Here are 3 commits that make minor modifications that are used in the PRESTO running on guillimin.

One adds a check to single_pulse_search.py for infinite cand sigma, which is sometimes the case with RFI, and doesn't plot it if cand.sigma is infinity. This prevents it from crashing in that case.

Another changes spectra_per_subint in sigproc_fb.c so that it is divisible by more downsample factors.

Another stops printing a message when zapping birdies because with dynamic zaplists, which have a lot of short ranges to zap, it causes a lot of output to stdout which leads to prohibitively large output log sizes.

Let me know if you have any questions.

Paul