Blank importance objects can't be printed

[MicahGale]

Describe the bug

Importance is missing a default _tree and when str is called this leads to an AttributeError.

To Reproduce

A short code snippet of what you have ran. Please change or remove any specific values or anything that can't be public. For example:

problem = montepy.read_input("tests/inputs/test_importance.imcnp")
print(problem.cells._importance)

Error Message (if any)

If an error message was printed please include the entire stacktrace. If it includes any specific values please change or remove them. For example:

    print(self)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = Importance: in_cell_block: False, set_in_cell_block False,
{}

    def __str__(self):
        if not self.in_cell_block and self._problem is None:
>           return " ".join(self.input_lines)
E           AttributeError: 'Importance' object has no attribute 'input_lines'

montepy/data_inputs/importance.py:165: AttributeError

MCNP input file snippet

see tests/inputs/test_importance.imcnp

Version

• Version 0.4.1