Update multithread.py
Merge pull request #235 from joshuasims/patch-2

Update multithread.py
i242: adjacent geometries result in subset error when output requires wrapping

A deepcopy was introduced before a the spatial dimension is wrapped.
The reference to the base field was being updated resulting in adjacent
geometries throwing subset errors.

Closes #242.
i232: ensure osr will properly raise exceptions

Closes #232.
Merge branch 'master' into i183-units