Plane class

# TODO

- [x] test coverage
None of the other geometry classes (for reasons than being an abstract base class) had to be XFAILed. Try to find out what they are doing to their args that you are not and make sure that for plane P, P.func(*p.args) == P.

Also, could you place a double space before the newly added functions, e.g. `def test_sympy__geometry__line3d__LinearEntity3D():` should have a double space before its XFAIL header; the the functions following it should also have double spaces before them.