Modifications in the replica exchange algorithm.
Chhanged the behaviour of the set-accessor for the simulation parameters. There will be no more reset of the simulation parameters in the accessor now.
Chhanged the behaviour of the set-accessor for the simulation parameters. There will be no more reset of the simulation parameters in the accessor now.
Changes in the documentation and the structure of the class for optimal ensemble sampling.
Removed some unnecessary functions in the optimal ensemble sampling. There is a clearer structure if the user does this simple functions.

Closes #76
Added a function to negate a histogram. Added the corresponding test functions.
* Updated the documentation of the optimal ensemble sampling class.
* Included 3 functors for calculating the iteration steps from the iteration index and the initial steps per iteration
* Included the possibility to use user-defined iteration number functions.
Merge branch 'master' into WangLandauParallel