Added basic tests for adding pandas data structures.
Added basic tests for adding pandas data structures.

Would be nice if we can write one set of tests for all of the four
structures and have these run against each one by pytest. Will look into
this to simplify things.

datreant/datreant.core,datreant/datreant.core,datreant/datreant,andreabedini/datreant,datreant/datreant.data,dotsdl/datreant,datreant/datreant