FIX: Grid object

Fixing a minor bug in the `Grid` object as to how it handles fields with incorrect shapes. Also adding the capability for the user to exclude fields during a read and an `add_field` method to the Grid object, both of these being similar to their counterparts in the `Radar` object. Finally, these additions allow for backwards compatibility with previous versions of MMCG @scollis created and some of the ARM standards.