Swap access and path argument order of Arg

Make op2.IdentityMap optional for Dats, closes #67
Update docstrings
Purge IdentityMap and replace with None
Ignore optional path argument when building Arg from Global
Add unit tests for creating Arg from Dat
Add unit tests for creating Arg from Global
Add unit tests for creating Arg from Mat
Merge pull request #214 from OP2/identitymap

Remove the IdentityMap from the API and swap the argument order for
Arg construction.  It's now obj(intent, map).
Bump version to 0.2.0