I'm a bit concerned this isn't doing quite what it says on the tin.
I understand that we need cfchecker because test_pp_cf uses it.
But as far as I can see, it does **not** use its own copy of cf-standard-names.xml by default
  -- instead, it will fetch it from ```STANDARDNAME = 'http://cf-pcmdi.llnl.gov/documents/cf-standard-names/standard-name-table/current/cf-standard-name-table.xml'``` (from cfchecker code)
In which case, it seems odd that this works at all, as it is ??

Meanwhile, what we used to do here was
 1. download a specific table and push it into lib/iris/etc
 2. create a custom cfchecker that refers specifically to that

I don't think we should be ditching that approach entirely here, so can we **at least keep the old version commented out** ?

I'm puzzled as to how the cfchecker test can be working now, if cfchecker is *itself* downloading from pcmdi.
As it happens, though, we do already have a copy of the version 23 'cf-standard-names.xml'  **burnt into the Iris repo** at lib/iris/etc
So I think it would be better to -
 1. comment out the xml download for now
 1. revert to the previous method of providing a 'custom' cfchecker, where the source is loaded from pypi