Deal with thread-safety issues

[ajdawson]

Many people use biggus underneath iris to work with NetCDF data. If an HDF5 library that is not thread-safe is used, erros can be generated (e.g., SciTools/iris#2063). This could be dealt with in biggus by using locking (as in xarray) or by providing a non-threaded engine that can optionally be selected by the user at run-time.

[cpelley]

What does this mean for the move to dask, can this be captured as a requirement for iris dask usage also?
ping @bjlittle