Allow saving non-ascii strings to netCDF with no '_Encoding' attribute

[pp-mo]

While writing the detail account in #7194, it has struck me that one usage that you can't get with the current proposal is to "automatically" encode data with utf-8 in the absence of an specific _Encoding attribute, and so to save without an _Encoding attribute.

So, I believe that is correct + ok according to the latest CF Conventions (>=v1.12).
But with the current proposal, Iris will insist on having an encoding attribute, and saving with one.

• From the latest CF (post-v1.12), such data is certainly "ok"
• .. and Iris will load it without question
• .. but Iris refuses to save like this without also adding the _Encoding attribute
• so, such data won't directly read-write roundtrip (will fail on re-save),
  and requires the user to add an encoding attribute to make it save

Technically, with the code as it is, if we were simply to change iris.fileformats.netcdf.DEFAULT_WRITE_ENCODING to "utf-8", then I think all the above problems would be solved :

• unicode data with no encoding attribute will automatically save as utf8, with no encoding attribute created
• both ascii and utf8 data don't need an "_Encoding" attribute, for either read or write
  but can have one if wanted.
• changes to Chardata docs #7194 will be needed -- but I think it all gets simpler