Adapt "is_canonical" method to contemplate several formats of canonical expressions

[jmansilla]

The is_canonical method should take an optional argument named "mode". By default mode should take the value "eager".

An expression is in canonical form if one of the following is met:

• the root node is an instance of Lam
• the root node is an instance of BooleanConstant
• the root node is an instance of NaturalConstant
• the root node is an instance of UnaryOperator who's operation is "-", and it's operand is a NaturalConstant
• in mode "normal", if the root node is an instance of Tuple
• in mode "eager", the root node is an instance of Tuple, and every sub-expression of the tuple is in canonical_form as well

Tasks:

• adapt method
• add correspondent tests cases