Allow filtering PyMJCF find_all results by default class

[sylvesterkaczmarek]

Addresses #482.

Changes

Add an optional dclass string argument to PyMJCF Element.find_all. It filters the found sites (or other namespace elements) by their explicit default-class name without requiring callers to handle missing class references or traverse element.dclass.dclass manually.

• Resolve the class name from PyMJCF's default-class element reference, handling absent references.
• Apply the filter recursively, including nested bodies.
• Preserve all existing find_all behavior when dclass is omitted.
• Add regression tests for matching, nonmatching, missing-class, recursive and immediate-children cases.

Validation

dm_control/mjcf/element_test.py: 47 passed and 2 subtests passed. Python compilation and git diff --check passed. Generated binding files were used only in the local test environment and are not included in this PR.