Exclude .git and __pycache__ directories from flake8 test
Allow setting configuration options via environment variables

The following variables are respected and translated into
configuration keys:

* PYOP2_DEBUG -> 'debug', type int
* PYOP2_LOG_LEVEL -> 'log_level', type str
* PYOP2_BACKEND -> 'backend', type str
* PYOP2_DUMP_GENCODE -> 'dump-gencode', type bool
* PYOP2_DUMP_GENCODE_PATH -> 'dump-gencode-path', type str
Bypass instant cache for host code generation if debug is enabled
Remove redundant vector declarations in OpenMP
Set, DataSet, Dat, Global, Mat and Map yield self when iterated over
Add unit tests for Set/DataSet/Dat/Global/Mat/Map iteration
Set, DataSet, Dat, Const, Global, Map have len 1 and tests for it
Add MixedSet type
Add unit tests for MixedSet
Add MixedDataSet type
Unit tests for MixedDataSet
Add MixedDat type
Unit tests for MixedDat
Add MixedMap type
Unit tests for MixedMap
Add mixed types to sphinx user docs
Build a sparsity block for each pair of DataSets in the mixed case
Implement Sparsity __getitem__, shape, __iter__
More Sparsity unit tests, mostly for mixed case
Add unit test creating a Mat from a mixed Sparsity