Requirements re-haul and optional testing against Iris latest

[trexfeathers]

This PR:

• Replicates the requirements structure of Iris introduced in Requirements re-haul iris#3812
• Re-introduces the option of testing against the latest iris source code, which was previously removed in Test against latest conda-forge iris. #100
  The Travis matrix now tests against the latest release, and the latest source code

As discussed offline with @pp-mo, my conflicting time commitments have stopped me testing this, so we have agreed that testing can be part of a more thorough review process before it's appropriate to be merged.

[trexfeathers]

@pp-mo not planning to do any more work on this. Please take a look and see how we can get the tests working!

I also haven't looked too closely about whether the pip set up is OK. One question that springs to mind is whether wheel should be part of requirements/setup.txt.

[pp-mo]

Replaced by #231 : on new feature-branch
( Most of the code there is actually derived from this. )