Switch to using DimCoord.from_regular

This PR converts usage of `f.regular_points()` within the PP rules into a new factory method `DimCoord.from_regular()` (via SciTools/iris-code-generators#9). By controlling the creation of the points and bounds arrays, the factory method can bypass some of the sanity checks and provide a performance boost.

Approx 3% performance boost doing iris.load_cubes() on a 17MB PP file containing 2048 fields.