Wrong Error Message Timers.py

[ge75buc]

Dear PyTrees-Team,

in line 59 of timers.py (https://github.com/splintered-reality/py_trees/blob/devel/py_trees/timers.py#L59) the error message suggests, that the input can be of type float OR int, while the actual check only accepts type float.

While the behaviour is correct, the error-message may be reformulated to: "Timer: duration should be float, but you passed in {}".format( type(duration)