Add methods to BaseGrid

[dougiesquire]

There are a number of commonly implemented methods that are not in the BaseGrid. Adding these as NotImplemeted methods in BaseGrid will make it clearer what can/should be added to grid types:

• @classmethod fromfile
• write - I think it could be a good idea to have just a single write method on each grid type to write the grid nc file(s), with kwargs specifying options for a specific grid type (e.g. whether or not to write a mask file also). This would mean the interface for writing grids would be the same for all files, which is currently not the case (e.g. OasisGrid has write_grids, write_areas and write_masks methods).

Also:

• write_mesh see Move mesh generation functionality into esmgrids? om3-scripts#11