Fixes for algebraic cudd

[CazSaa]

This pull request includes several changes to the dd/cudd_add.pyx file.

Changes to function definitions and error handling:

• Changed the return type of Cudd_addLeq from DdNode* to int to reflect its actual return type. (The library did not compile for me without this fix)
• Replaced the implementation of comparison operators (<, <=, >, >=) with NotImplementedError because the implementation does not work with this return type.

Enhancements to file type support:

• Added support for the 'dot' file type in ADD.dump.

Improvements to the Function class:

• Added a value property to the Function class to return the value of a leaf ADD node.

Modifications to _to_dot_recurse function:

• Updated the label for leaf nodes to display their value instead of 'TRUE' or 'FALSE'.
• Removed taillabel attributes from edges in the _to_dot_recurse because it is redundant with the edge style.

Remove erroneous shortcut in find_or_add:

While i was playing around with this library I found out that for an ADD i constructed, the structure was different from the one I created using PyCUDD. I traced it down to this erroneous shortcut. I'm guessing you tried to mimic the behaviour of one of the shortcuts that is implemented inside CUDD, but i think there was a mistake.

[johnyf]

Thank you for finding these issues, I will review the changes.