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.

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

FUlly reorganised theory files. All seems functionning. Theory files now all simply have to extend proven.Main, and get every import and tool.
Documentation for recent development.
Adaptation of existing Set Theory proofs to the new canonical style.

Printing, constructing theorems and all kinds of definitions, authomatic conversions, and much more.

Printer file moved to utilities
New organisation of helpers, new helper file for RunningTheory and their elements. Can show Justifications
SCProofCheckerJudgement now contain the proof they refer to, no need to pass it explicitely along anymore.
Other small changes.