Issue #7537: Add is_disjoint() method and an alias isdisjoint()

Add a method for finding if two sets are disjoint.
I think it does.  I'm guessing based off of [the docs for Relational](http://docs.sympy.org/latest/modules/core.html#module-sympy.core.relational), which do show `Rel` etc. even though [they're just class aliases](https://github.com/sympy/sympy/blob/master/sympy/core/relational.py#L144).  I would hope Sphinx does the same for method aliases, but I don't know for sure.