dfply tool tests need work: - Many of the tests seem redundant; can we reduce to a minimal set? - Some of the dfply tool files have very low coverage: [`set_ops.py`](https://codecov.io/gh/zdelrosario/py_grama/src/master/grama/dfply/set_ops.py), [`subset.py`](https://codecov.io/gh/zdelrosario/py_grama/src/master/grama/dfply/subset.py), [`window_functions.py`](https://codecov.io/gh/zdelrosario/py_grama/src/master/grama/dfply/window_functions.py)
dfply tool tests need work:
set_ops.py,subset.py,window_functions.py