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