grad/div/curl operators don't take `side` as a kwarg that can be passed to the components

[cdelavegamartin]

This is a request/improvement to pass extra kwargs (like side) to the components of operators like grad div and curl
Current behaviour for side,

from devito import Grid, Function, VectorFunction, centered, curl

grid = Grid(shape=(11, 11, 11))
f = Function(name='f', grid=grid, space_order=2)
f_vec = VectorFunction(name='f_vec', grid=grid, space_order=2)

expr1 = f.grad().evaluate
expr2 = f.grad(order=2, side=centered).evaluate
display( expr1 )
assert expr1 == expr2

expr1 = f.div().evaluate
expr2 = f.div(order=2, side=centered).evaluate
display( expr1 )
assert expr1 == expr2

expr1 = curl(f_vec).evaluate
expr2 = curl(f_vec,order=2, side=centered).evaluate
display( expr1 )
assert expr1 == expr2

$$\left[\begin{matrix}- \frac{f(x, y, z)}{h_{x}} + \frac{f(x + h_x, y, z)}{h_{x}}\\- \frac{f(x, y, z)}{h_{y}} + \frac{f(x, y + h_y, z)}{h_{y}}\\- \frac{f(x, y, z)}{h_{z}} + \frac{f(x, y, z + h_z)}{h_{z}}\end{matrix}\right]$$ $$\left(- \frac{f(x, y, z)}{h_{x}} + \frac{f(x + h_x, y, z)}{h_{x}}\right) + \left(- \frac{f(x, y, z)}{h_{y}} + \frac{f(x, y + h_y, z)}{h_{y}}\right) + \left(- \frac{f(x, y, z)}{h_{z}} + \frac{f(x, y, z + h_z)}{h_{z}}\right)$$ $$\left[\begin{matrix}\left(\frac{f_vec_z(x, y, z + h_z/2)}{h_{y}} - \frac{f_vec_z(x, y - h_y, z + h_z/2)}{h_{y}}\right) - \left(\frac{f_vec_y(x, y + h_y/2, z)}{h_{z}} - \frac{f_vec_y(x, y + h_y/2, z - h_z)}{h_{z}}\right)\\- (\frac{f_vec_z(x, y, z + h_z/2)}{h_{x}} - \frac{f_vec_z(x - h_x, y, z + h_z/2)}{h_{x}}) + \left(\frac{f_vec_x(x + h_x/2, y, z)}{h_{z}} - \frac{f_vec_x(x + h_x/2, y, z - h_z)}{h_{z}}\right)\\\left(\frac{f_vec_y(x, y + h_y/2, z)}{h_{x}} - \frac{f_vec_y(x - h_x, y + h_y/2, z)}{h_{x}}\right) - \left(\frac{f_vec_x(x + h_x/2, y, z)}{h_{y}} - \frac{f_vec_x(x + h_x/2, y - h_y, z)}{h_{y}}\right)\end{matrix}\right]$$