Adding simplify step after dividing by a singfun

- singfun/rdivide now matches singfun/times, per discussion in #1006
- updated comment in simplifyExponent
- added test to ensure that rdivide deals with exponents as intended

As I started writing the test, I couldn't help but think about "auto"-simplification.  That is, if we want singfun objects (or objects of other classes) to, generally, stay tidy, do we want them to be able to exist outside of that state?  

Also, I noticed that when we simplify a singfun whose exponents are both greater than or equal to 1, we may (or always?) end up with an object that's not a singfun.  I don't how exactly this fits into the bigger picture but you can see the probably inelegant way that I handled this in line 104 to 108 of the test below.