new branch for tricky echosounder work
echosounder steppingstone: RLE early steps

Checking in mainly to thin out the diffs, which are large because of
going back on forth on the idea of handling all three cases in one
function.
echosounder: start renaming things in C code
echosounder seems OK on split-beam data

At least, it produces sensible-looking results on the 3 test datafiles
from biosonics.  NOTE: conversion to dB not done yet.  I'll get
single-beam working first.
echosounder: works on single-beam now also
echosounder: improve docs
Merge branch 'echosounder' into develop