Fixing inconsistent formatting / naming, and reducing some duplicated code

* The variable arguments "I1" and "I2" are meaningless without reading the inline parameter comments. These were changed to have meaningful names: "i_max" and "i_min" respectively. 
* The P,I,D variables in arguments were changed to be more consistent with the rest of the API: p,i,d
* Some duplicated initialization code in the constructor was removed, and replaced with a call to the `reset()` function (which does the same thing as the removed code).
* Some duplicated member-setting code in `initPid` was removed, and replaced with a call to the `setPid()` function (which does the same thing as the removed code).

This PR changes neither the API nor the behavior of this library.