Add abstract methods, read & write to the AbstractVariable
old parameter 'value' renamed in 'originalValue'
Add 'name', 'learnable' and 'mutable' properties and abstract methods 'read', 'write' and 'isDefined' in AbstractVariable.
Implement the 'read' and the 'write' method for leaf variables in AbstractVariableLeaf.
Implement isDefined and getValue plus some fixes in docTests in Data.
Implement 'read', 'write' and 'isDefined' methods for Alt and Agg Variables.
small fix
Add the 'incrementIndex' method in VariableReadingToken.
add some imports in __init__
fix one doctest
add more doctest