Fix gcylc zero division error from task mean elapsed time

[sadielbartholomew]

An (internal Met Office operational) user has noticed a traceback dialog in the GUI:

Traceback (most recent call last):
  File "<HOME>/cylc.git/lib/cylc/gui/updater_tree.py", line 257, in update_gui
    100 * (tnow - tstart) / (tetc_unix - tstart))
ZeroDivisionError: float division

A brief look at the code reveals this is due to tasks which have a mean elapsed time value of zero - quite how this has occurred in a real suite is still a bit of a mystery to me and while the fix was trivial and the logic easy to follow, the difficultly here was reproducing the error (for self-review purposes) i.e. setting up a suite so that at least one task would have mean elapsed time of zero.

Eventually I just did a bodge job, so (unless you can easily come up with a more clever idea!) to recreate, just change line 240 to meant = 0.0 and the if tnow > tetc_unix statement on line 253 to if False to automatically proceed to read the line in question, then run some suite with at least one task with script sleep 10 or similar that cycles at least once (so meant gets re-computed).

N.B. there is some logic on 280:283 which does change meant to 1 if it is in fact 0 and I initially tried essentially moving that above the conditionals in question so the zero division error would not ever get raised, but it does influence the conditional on 261:264 so this simple change seemed more appropriate.

[hjoliver]

Thanks @sadielbartholomew