Logics

Added
- Shortcut for importing the classical predicate parser

Changed
- Small change in the project's README

Added
- Extra parameter ``exit_on_first_error`` for Tableaux and SequentCalculus
``is_correct_tree`` mathod, and AxiomSystem's ``is_correct_derivation``.

Changed
- Every module's ``is_correct_xxx`` method now returns a list of
``logics.classes.errors.Correctionerror`` objects if asked to return the error list.

Added
- Indexed presentation of classical logic tableaux.

Changed
- Renamed ``ManyValuedTableauxSolver`` and ``mvl_tableaux_solver`` to
``IndexedTableauxSolver`` and ``indexed_tableaux_solver``

Fixed
- Bug fix in predicate parser, now correctly parsing sentences with sentential
metavariables.

Changed
- Quantified formulae given to the predicate parser should not contain parentheses
(e.g. `"∀x P(x)"` instead of `"∀x (P(x))"`)

Added
- Predicate natural deduction.

Changed
- Predicate languages now accept individual and variable metavariables
(they are useful for formulating rules).
The relevant `PredicateFormula` methods were changed to account for this.