Start drafting the addition of an optional coordinates argument to the
grid method of the BaseGridder.
It would allow use to predict on preexisting grids and obtain
a xr.Dataset as output.
Add a new example that passes the coordinates of a preexisting grid to
a Chain with a Spline inside.
Write a new meshgrid_from_1d function that creates a meshgrid out of 1d
horizontal arrays, but also checks if they have a single dimensions and
supports for passing extra coordinates as 2d-arrays.

… allow-coords-in-grid

Rename variable to avoid overriding the grid_coordinates function

Co-authored-by: Leonardo Uieda <[email protected]>

We will change every gridding example so they predefine grid coordinates
and pass them as the `coordinates` argument.

Co-authored-by: Leonardo Uieda <[email protected]>