Harmonize the C++ solvers API

[fteicht]

This PR :

• adds callbacks to all the C++ solvers and their Python class interfaces
• changes the signature and name of previous named "watchdogs" to "callbacks"
• adds getter methods in the C++ solvers to be able to monitor some solver attributes from C++ and Python within the callbacks
• changes the "debug_logs" parameter in the solvers' constructors to "verbose" in order to harmonize with the pure Python solvers
• corrects a few bugs in the solvers