Adding XLinear_Velocity interpolator

[erikvansebille]

This PR adds an explicit XLinear_Velocity interpolator, effectively moving the code from inside field.py. This improves transparency what is being done in interpolation.

Parcels/src/parcels/_core/field.py

Lines 307 to 315 in eeedce8

	if self._vector_interp_method is None:
	u = self.U._interp_method(particle_positions, grid_positions, self.U)
	v = self.V._interp_method(particle_positions, grid_positions, self.V)
	if "3D" in self.vector_type:
	w = self.W._interp_method(particle_positions, grid_positions, self.W)
	else:
	w = 0.0
	else:
	(u, v, w) = self._vector_interp_method(particle_positions, grid_positions, self)

First of all, do you agree this would be a good change?

A few more more things to do

• base the choice between Grid_Velcoity and XLinear_Velocity would be based on grid metadata, in FieldSet creation. @VeckoTheGecko, could you help?
• Make similar functions for UxGrids. @fluidnumerics-joe, could you pick that up?
• Remove the code linked above from field.py (so that VectorFields can't have _vector_interp_method=None