reset functionality for stateful position limit handles

As discussed in #193 I have added reset functions to the stateful position limit handles.
Existing API was not touch, I have just added new methods to the handles and the corresponding interfaces.
I think strict compilers will complain of a missing `typename` here. It should read `typedef typename hardware_interface...`

The above statement designates a nested type of a templated class, and compilers need help in figuring out that we're actually talking about a type here.