Mutwo-common

Added
- explicit exception classes in `mutwo.common_utilities`

Changed
- raised exceptions to custom exception classes
- package name from `mutwo.ext-common-generators` to `mutwo.common`

Added
- `Backtracking` and `IndexBasedBacktracking`

Added
- property `terminal_tuple` and `non_terminal_tuple` to `ContextFreeGrammar`

Added
- property `context_free_grammar_rule_tuple` to `ContextFreeGrammar`

Added
- chomksy submodule with support for context-free grammar

Changed
- from `expenvelope.Envelope` to `mutwo.core_events.Envelope`

Removed
- `expenvelope` dependency

Changed
- Apply refactor from first plugin to namespace package plugin structure