Get the unit-type switch working

[pp-mo]

Note: for choice, not before #6999

At present, this is set to make all units pint-type (for test equivalence testing).
This should change so that both types of unit can co-exist (inside Iris).

In the PoC this happens here
(at least, I think so).

Desired behaviour is ...

• either of the inbuilt wrappers should produce a copy
• other pint or cf_units should be wrapped
• a string (or other thing that converts to a string) should be created as the default type set by XXXX

.. and test --> see #7007

Extra scope

It seems sensible to me to replace the default_units_class() function with a simple make-unit() call
-- without changing the scope or function, that is.
It is only actually used in one place, so that should be pretty simple.
So, that's what we'll be testing now -- but with various types of unit inputs.
(i.e. as for "Desired behaviour", above)

[pp-mo]

Since I'm assuming this elsewhere, here's my simplistic idea :

def make_unit(
        arg: None | str | cf_units.Unit | pint.Unit
) -> CfUnit | IrisCfulikeCfpintUnit:
    """Convert input into an Iris unit.
    
    Converts strings to units, and pint/cf_units Units to the Iris specialised
    derived unit types .
    """
    unit_class = _default_units_class()
    return unit_class.from_unit(arg)

Feel free to adapt as you see fit.

[pp-mo]

#7029 fixes