Remove Wang-Landau Reaction Ensemble algorithm

[jngrad]

The current Wang-Landau Reaction Ensemble (WLRE) implementation is tightly coupled to the other reaction methods in the C++ core. This accidental complexity slows down the development and improvement of the other reaction methods. Rewriting the WLRE method to improve modularity would be a significant project for the core team. This reaction method is not actively used by the community, and a recent call on the ESPResSo mailing list did not receive any response against the removal of the method:
https://lists.nongnu.org/archive/html/espressomd-users/2021-06/msg00001.html

Partial fix for #4251 (removing WLRE hooks)
Closes #4156

Description of changes:

• Remove the unused WLRE method. It is not actively maintained and not fully tested (86% code coverage).
• Fix undefined behavior in ReactionAlgorithm (member particle_inside_exclusion_radius_touched was uninitialized).
• Write additional unit tests.