Starting to implement BackToBackExponentialPV. Refs #8744.

Starting to implement BackToBackExponentialPV.
The code is migrated from BacktoBackExponential.
Some changes have been implemented.
But it is not finished yet.
Finished implementation of the function. Refs #8744.
Added some unit tests. Refs #8744.
Fixed unit test. Refs #8744.
Modified doc in wiki. Refs #8744.
Added one more unit test. Refs #8744.
Merge branch 'feature/8744_bk2bk_exp_pv' into develop