ENH: Cached cropped_bounds within regrid

See https://github.com/SciTools/iris/pull/1431 for further details.

Going from 30second (21600, 43200) source to N96 (145, 192):
Unoptimised: 43s
Optimised: 26s (does not include #1431)

Combined with #1431, offers a double in performance of regrid for the above.

When regriding from a 15seconds field to N2048 (decomposed to 30 pieces on multiple hosts using MPI) the combined optimisation of these PRs takes this from **20minutes to 3.7minutes!**