cleaned filtration time

* filtration time is now fixed inside each range, instead
  of trying to follow linear curve
* don't bother being super accurate, that's too much
  over-engineering after all ;)

pool.js

@@ -146,6 +146,7 @@ function do_call(calls) {
 }
 
 // compute & configure pump schedule
+// TODO force on  when duration==24
 // TODO force off when duration==0  
 // TODO freeze mode if cur < 1