Stop reliance on hashable args

[Carwyn Pelley (cpelley)]

A set is used to perform argument checking logic. Using a set enforces a requirements that the arguments are hashable (which they may not always be).

The relevant line:

dagrunner/dagrunner/execute_graph.py

Line 106 in 0b98a63

if set(args) == {IGNORE_EVENT}:

We can recreate the exception manually:

>>> set([iris.cube.CubeList([iris.cube.Cube(0), iris.cube.Cube(1)])])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'CubeList'

[Stephen Moseley (MoseleyS)]

This branch fixes the issue we've recently uncovered with handling unhashable CubeList objects

[Carwyn Pelley (cpelley)]

It's now out of draft and ready for review Rob Purvis (@mo-robert-purvis), Stephen Moseley (@MoseleyS)