add threshold options to sourcefind output

[twshimwell]

Presently thresholds (thresh_isl=3.0, thresh_pix=4.0) are hardcoded. We can of course make this optional (for the deep fields we thought to use thresh_isl=3.0, thresh_pix=4.0 but for LoTSS we thought to perhaps use thresh_isl=3.0, thresh_pix=4.5). However, now the latest master pybdsf outputs the wavelet rms and wavelet mean values (in the component catalogue). So in theory we can simply run with whatever thresholds and cut the catalogue afterwards.

For completeness it might be nice to include these post pybdsf cuts in the cataloguing steps after the source finding. For LoTSS we discussed cutting at thresh_pix=5 but its nicer to run pybdsf with a lower thresh_pix because then you get sources that pop over the 5 sigma threshold that would have been missed if thresh_pix=5