Fast add coords

[rhattersley]

This PR has two main aspects:

1. It modifies the PP/GRIB load rules (via SciTools/iris-code-generators#8 which should be merged first) so all the coordinates are collated instead of added directly to the Cube. (Similar changes are also applied to non-coordinate metadata.) The coordinates are then supplied to the Cube constructor.
2. It speeds up the addition of coordinates during the Cube constructor by using a quick uniqueness check where possible instead of the full, slow self.coords(coord=coord) check done by Cube.add_dim_coords() and Cube.add_aux_coords().

Approx 7% performance boost doing iris.load_cubes() on a 17MB PP file containing 2048 fields.

[rhattersley]

Since this branch now has merge conflicts and no one has commented yet I'll rebase ...

[bblay]

Would you consider moving this logic into add_dim_coords and add_aux_coords so other code can take advantage of it?

[bblay]

hmm, actually it's not so applicable outside here.

[rhattersley]

This logic only works when adding a bunch of coordinates in a single operation, so it isn't suitable for either of those methods. It's not possible to have a simple, persistent cache of identities simply because everything about a coordinate's identity is mutable!

[bblay]

yep

[rhattersley]

If there's nothing else, are you happy for me to rebase now?

[rhattersley]

Rebased from 418156c6664085a5344c85049f849b9159669bc2 to 336e4a1.