ENH: Grid object

Adding the ability to exclude fields during the creation of a `Grid` object, as well as the ability to add a new field with the `add_field` attribute similar to the `Radar` object. Furthermore, I am adding some extra axes and dimensions to check for when creating a Grid object. These axes and dimensions are compatible with older versions of MMCG files. Finally, this corrects a bug during the creation of a Grid when a field has the wrong shape.