Varstrs

[pp-mo]

Closes #223 (treating that as an issue)

WIP currently based off #253

Starting point code.

• use arrays of type object (dtype "O") to represent variable-length string data
  • ... as netCDF4 does
• support read and write to netCDF4
  • elements are all type "str", but we can't easily/quickly verify that
  • when writing, use .astype("U"), so can write ~any objects-- but obviously, won't round-trip
  • when reading, special control ncdata.netcdf4.ASSUMED_TYPICAL_STRINGLENGTH controls chunking

The code changes are surprisingly small + simple.

Todo:

• read/write to Iris : presumably needs similar support in Iris itself -- cf Support variable-length strings iris#7092
• read/write to Xarray
  * N.B. Xarray already supports them?
• tests
• documentation (many changes needed to existing?)
• ASSUMED_TYPICAL_STRINGLENGTH should probably be inside a threadsafe setting/context object.