High-resolution regular coords

A user case revealed that some quite simple regular coordinates are judged not regular by Iris.
  -- see the testcase added in the first commit :  These points are just ```355.626 + 0.0135 * np.arange(37)```

It was suggested (by @pelson ? can't remember) that we can't reasonably represent the fact that a coordinate values were "supposed to be" regular in Iris, but should instead depend on the point values, as this is basically what the CF data model imposes.

So this change makes the result of Coord.as_regular have high-resolution (float64) values.

The basic change is just 2 characters, but a lot of test references (files and literals) are affected.
Those changes are in a separate commit.