Solver factory: correct naming scheme and typos

Apply naming schemes to solver factory function, improve some internal variable names and correct some spelling and grammar mistakes.

The most severe issue fixed:

• the functions default_preconditoner_BL_creator and default_preconditoner_creator didn't comply with the naming scheme and also had a spelling mistake: missing i in preconditoner.
• in the usage of registry_put the name didn't make it clear that it is a macro. Also the macro should definitely be prefixed with DUNE to avoid conflicts with other libraries -> changed to DUNE_REGISTRY_PUT.

@gruenich Should probably be fixed before the release 2.7 so we don't need to deprecate the names.