Add tests for sequent parsing.

Implement a parser for first order logic with equality and schematic functions/predicates
using scallion.

Require that PredicateFormula's and ConnectorFormula's args correspond to the label's arity

General grammatical updates for reference manual

PeanoArithmetics contains the definitions and axioms of Peano arithmetics,
PeanoArithmeticsLibrary gives access to lisa.utils.Library with runningPeanoTheory,
Peano contains proofs of the theory. This separation follows the same structure
as set theory.

Analogous to the existing requirement in FunctionTerm.

Fix a broken link to the reference manual

Signed-off-by: Sankalp Gambhir <sankalp.gambhir47@gmail.com>

Add utils to follow the path to an incorrect proof step and to create an error message from invalid proof judgement

Now variables are always shown with a leading ? (except when following a binder)

Any work on top of current LISA release should be easily adaptable by:
 - Changing all instances of Arity 0 SchematicFunctionLabel by VariableLabel
 - Changing all instances of Arity 0 SchematicPredicateLabel by VariableFormulaLabel
 - Changing all matching on SchematicFunctionLabel by either VariableLabel, SchematicFunctionLabel or SchematicTermLabel, according to whether the arity should be =0, >0 or >=0.
 - Changing all instances of multiary SchematicPredicateLabel by SchematicNPredicateLabel
 - Removing the empty parenthesis () that where used to transform an Arity 0 label to a term or formula. Variables have the implicit conversion without the call.

Changed the whole project to remove 0-arity schematic variables and replace them by variables. It compiles, but at least one proof doesn't go through.

Changed final class into sealed class for scala2 compiler