Merge branch 'develop' into echosounder
swSoundAbsorption() added (2 formulations)

Want this for read.echosounder() (or other function) to convert to dB
etc.
Merge branch 'echosounder' into develop
read.echosounder(): start coding "Sv" support (broken)
imagep(): prevent problem if z contains +- Inf
echosounder: use dB for sl, rs, tpow and corr
echosounder: use proper unit for bwx in computing psi
read.echosounder(): improve calc of Sv and TS
Merge branch 'echosounder' into develop
plot.echosounder(): white out below-threshold data (issue322)
Merge branch 'echosounder' into develop
data(echosounder) updated to new format
Merge branch 'echosounder' into develop
plot.echosounder() new arg drawTimeRange
Merge branch 'develop' into echosounder
doc: improve accessor explanation
Merge branch 'echosounder' into develop
echosounder: permit assignment to e.g. [["Sv"]]
as.echosounder(): add sufficient args to calc e.g. Sv
add sufficient args to create e.g. Sv