Fixed analysis._interpolator handling of decreasing coords.

This fixes a previous legacy test failure in:
test_monotonic_decreasing_coord (iris.tests.test_interpolation.TestLinear1dInterpolation)

I'm posting this as-is
 -- but note that it causes a **new** test failure 
in test_interpolate_data
in iris/tests/unit/analysis/interpolator/test_Interpolator.py

I'm not *sure*, but I think this may be a mistake in the test routine.
Please consider carefully.