Extend grid object to cover cases of irregular Cartesian grid

[jfigui]

The current grid object deals only with regular Cartesian grids. However in many meteorological applications the grid is defined in terms of equal lat/lon spacing and therefore it is irregular in Cartesian spacing.

It should not be too difficult to adapt the code. The main issue is the role of the x, y, z parameters. In case of regular lat/lon grid this x,y,z should represent degrees instead of m from origin. We could use a boolean "ll_regular" i.e. lat/lon regular to define what those parameters represent.

The main issue here is to adapt all the code dependencies to treat the ll_regular=True case correctly