Use new Particle.add_variables method for creating Particle Classes

[erikvansebille]

Since parcels v3.0.2, we have a new method to create Particle Classes (Parcels-code/Parcels#1501)

So statements like
https://github.com/OceanParcels/Virtual_ship_classroom/blob/b9a1dbffb1e0389bf02e14babfb732d677efa157/virtualship.py#L158-L161
Can be changed to

ADCPParticle = JITParticle.add_variables([
        """Define a new particle class that does ADCP like measurements"""
        Variable('U', dtype=np.float32, initial=0.0),
        Variable('V', dtype=np.float32, initial=0.0)])