Remove GridSet class from Public API

[VeckoTheGecko]

I don't think the GridSet object really needs to be called by users at all, as they are not manually defining gridsets. All the grid information is available to the users via the Field.grid attributes, and FieldSet.gridset attributes.

I also think that the GridSet itself can be majorly simplified (ie, add_grid() removed/majorly refactored, grids being an actual set, turning it into a dataclass) to the point that we could maybe even outright replace it with some utility functions/processors that pull grid information from the Fields. That's out of topic for this issue, just a hint to where I can see this going.

@erikvansebille any insight onto the history of the gridset class?