BUG: Bug in expressions evaluation with reversed ops, showing in series-dataframe ops (GH7198)
closes #7198
closes #7192

@cpcloud this is fine in 0.13.1 and broke in 0.14.0...any clue?
 I think this is the same issues as in #7192 (as the subtraction is not commutative)
 I think this is my fault. I added the op() calls for the rops. This particular issue was untested. 
 fessing up! :)

np....this tests this explicity 

took me a bit to figure out that sub/div are not commutative while add/mul ARE!
 Might be better to just remove the rops op($ calls than to add the reversed check
 Unless that isn't the issue. But I think that's the only thing other than the raise on bool  ops that has changed between 0.13.1 and 14
 python somehow seems to handle them correctly (e.g. it in-line reverses)

```
__rtruediv__(x,y)
__truediv__(x,y)
```

do the right thing!

whereas in numexpr the 2nd is backwards

I don't have any idea WHEN python decides to call the reversed ops, but no biggie, just added a flag and it should work
 It's done at the c level in pyobject_richcompare I followed it once for some Timestamp compare stuff. Good times 
 haha....ok...no worries
