Removing Field.gradient() function

[delandmeterp]

The former Field.gradient() function was based on np.gradient() function, building central difference scheme within the domain and forward or backward scheme on the field boundaries.
While this function could be useful for different applications, it was facing many limitations:

• This function was not valid for curvilinear schemes
• For C-grids, the effective resolution was twice as coarse as it could be if computed properly
• Close to masked areas (coast), spurious gradients could be computed
• It required to use np.array data for the Field objects, which is not the case anymore in Chunked fields operational #632

The Field.gradient function is then removed from Parcels. Obtaining the gradient of the field can still be achieved easily within a kernel, as it is done in some diffusion kernels:

    dx = .01  # for spherical coords, dx is in degrees
    Fp1 = fieldset.F[time, particle.depth, particle.lat+dx, particle.lon]
    Fm1 = fieldset.F[time, particle.depth, particle.lat-dx, particle.lon]
    dFdx = (Fp1-Fm1) / (2*dx)

Note in such configuration, the gradient on a spherical mesh is computed as [F]/degree.
If the gradient is needed in [F]/m, it is necessary to rescale it, by multiplying it by a factor
f = 1852*60 in the meridional direction or f=1852*60*cos(lat*pi/180) in the zonal direction.