Add step to worker interface sliders

[bruyeret]

@zjniu This PR adds another property that you can set for the worker interface, that is "step" If not defined, it will default to -1 (no step, maximum precision) For example:, this will force the "Diameter" slider to be an integer between 0 and 200 (defaults to 10)

'Diameter': {
    'type': 'number',
    'min': 0,
    'max': 200,
    'default': 10,
    'step': 1
},