Remove deprecated module interpolate.py

[DPeterK]

Closes #2644

[corinnebosley]

@dkillick You've got a 'from iris.tests.test_interpolation import normalise_order' left in iris.tests.test_analysis_calculus.py.

[corinnebosley]

Pending the one-line removal and then the tests passing, I'm happy with this. I had some concerns that some of the tests in the module you have removed were testing the not-deprecated interpolate code, but I checked and they don't, so it's all good.

[DPeterK]

To avoid importing iris.tests.test_interpolation.normalise_order in iris.tests.analysis.test_interpolate.py I've just deleted this test module. The tests contained in it fail locally regardless of whether I introduce normalise_order back into the test module or not, and we're going to need to delete this test module anyway as part of upcoming work, so I thought I'd get ahead.

[corinnebosley]

@dkillick Hang on a minute, are we getting rid of all iris.analysis.interpolate code and just keeping regrid stuff instead?

[DPeterK]

@corinnebosley Travis has gone green!! 😀 🎉

[corinnebosley]

Boom!