Simulation propagation distance being less than step size not caught in validation

[DavidDierickx]

Recreation:
Set propagation distance to 0.5e-6 in Beam Creator
Set step size to 1.e-6 in Simulation Setup
Attempt to generate simulation

[patrickcleeve2]

added check to step size, fixed issue with ui check on step_size, added test for check. in c2fd40f