Review new convention 'duration=0' on new style decompositions

[wvlothuizen]

as mentioned in the subject, https://github.com/QuTech-Delft/OpenQL/pull/433 introduces a new convention where 'duration=0' disables duration checking when loading the platform instructions. This issue is created because the full implications (especially when performing multiple rounds of decompositions) need further discussion beyond that in the PR mentioned.