Fix bugs & add tests/wf-alike util/lfreq, util/cstats

Fix div by zero bugs, sequint.add from empty bug, in stat try to document why of SIMD-vectorized routines better and ease actually getting that boost, add stat.range for both min & max in 1-pass.

Track cligen pm=" +- " to just "+-" change in cstats and add it to the nimble install. Also add lfreq to the nimble install. Add cligen dependency for the two utilities.