[Core] Adding missing checks to test_dofs and document it

[loumalouomega]

📝 Description

This PR adds testing for the IsFixed methods in test_dofs. Also adds a test for Free method.

In addition, the PR adds documentation to the following methods from test_dofs:

1. SetUpTestModelPart Method: A method called __SetUpTestModelPart is added to set up a test ModelPart with nodes and DOFs.

2. testDofExport Method: This test case checks the export of DOF information. It creates a ModelPart with nodes and DOFs, exports and manipulates DOFs, and performs various assertions.

3. testDofListValues Method: This test case focuses on DOF list values and manipulation. It creates a ModelPart with nodes and DOFs, creates a DOF list, sets and retrieves values, and performs assertions.

4. testDofListAppend Method: This test case checks the appending of DOFs to a list. It creates a ModelPart with nodes and DOFs and appends DOFs to a list, asserting the length of the list.

5. testDofListUnique Method: This test case examines making a DOF list unique. It creates a ModelPart with nodes and DOFs, appends multiple copies of the same DOF to a list, makes the list unique, and asserts its length.

🆕 Changelog

• Adding missing checks to test_dofs and document it
• Improve test with testing Free too