Support variable-length strings

[pp-mo]

As noted in ncdata#223, we can't easily load or save variable-length string arrays, but this has become quite desirable

• we already implemented a recommended user-triggered workaround mechanism
• they are valid CF ("A text string can be stored either in a variable-length string or in a fixed-length char array")
• they are handled by netCDF4, where the variable has a dtype of 'str' (or on write, a 'Sxx' other than 'S1')
• (a) on Iris load, they hit a glitch whereby the cf_var.dtype is not an np.dtype, so has no .itemsize
• (b) on save they fail in a similar way, as it fails to get a default fill-value

Ideally we would just have special code to support this 'exception' type in load + save.
I would hope to return to this after fixing SciTools/ncdata#223