Update `igrid` when adding a new Field

[erikvansebille]

igrid is not being updated in the code - so all the indexing is done on the last grid (perhaps leading to the change in hash that we saw elsewhere?) Should we resolve that in another PR?

Changes required:

FieldSet.__init__
   - make the field addition use `add_field` instead of plainly constructing the 
     dict of fields
FieldSet.add_field()
   - update fieldset.fields
   - update fieldset.gridset (also make this a normal variable instead of a property)
   - update field.igrid indexing the grid position in the updated gridset

Originally posted by @VeckoTheGecko in #2211 (comment)