fix: fix upper bound coming from isl ast

[skourta]

This pull request to tiralib/tiramisu/tiramisu_tree.py includes changes to improve the handling of upper bounds and adds new methods for generating ISL AST strings. The most important changes include modifying the upper bound calculation and adding methods to generate ISL AST string representations of the tiramisu tree.

Improvements to upper bound handling:

• Modified the from_isl_ast_string_list method to add 1 to the upper bound when converting it to an integer to account for the ISL's inclusive upper bound.

Enhancements to ISL AST string generation:

• Added a new method get_isl_ast_string to generate the ISL AST string representation of the tiramisu tree.
• Added a helper method _get_isl_ast_string_of_node to recursively generate the ISL AST string for each node in the tiramisu tree.

[Mascinissa]

Looks good to me.