API: velocity input is not in SI units

[ThomasCeulemans]

When inputting data into magritte models, we assume SI units for most data, which is fine. However, for the velocities, we demand them normalized compared to the speed of light (this is for convenience the formulae).
We should think about (a) either changing the api (not fully recommend, a this breaks existing code) or (b) implementing an alternative api in which we demand the units of all inputs (using astropy units for example).