Rename density_of_states to log_density_of_states

[bkrueger]

Rename the class members of the entropic sampling algorithm and the Wang-Landau algorithm:

density_of_states -> log_density_of_states

Accordingly change the get-accessors in these two classes and in class optimal ensemble sampling

get_density_of_states() -> get_log_density_of_states()

With these changes it gets clearer for the user what is returned by the algorithms.