Allow overriding default configuration from environment

Each default configuration setting has an associated environment
variable used to override the default if present in the environment
at configuration time.
Configuration respects PYOP2_LAZY, PYOP2_MAX_TRACE_LENGTH
Convert config environment variables to expected type
Move configuration to its own module
Merge pull request #267 from OP2/267-set-config-from-environment

Allow overriding default configuration from environment.

* Configuration respects PYOP2_LAZY, PYOP2_MAX_TRACE_LENGTH
* Convert config environment variables to expected type
* Move configuration to its own module