Refactoring contact functions + tests

[Ali-7800]

Fifth contact module PR mentioned in Issue
This PR includes:

• Refactored contact functions in joint.py and interaction.py
• Updated test_joint.py and interaction.py accordingly
• Moved contact force calculation functions from contact_forces.py into _contact_functions.py
• Moved tests in test_common_contact_function.py,test_specific_contact_function.py, and test_contact_wrapper_classes.py into joint.py
• Added tests related to rod-plane contact in test_contact_utils.py,test_contact_functions.py, and test_contact_classes.py

[codecov-commenter]

Codecov Report

Attention: 44 lines in your changes are missing coverage. Please review.

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Files	Coverage Δ
elastica/contact_forces.py	91.01% <ø> (+29.51%)	⬆️
elastica/joint.py	96.90% <100.00%> (+11.05%)	⬆️
elastica/interaction.py	76.04% <81.81%> (-12.97%)	⬇️
elastica/_contact_functions.py	82.99% <82.99%> (ø)

... and 1 file with indirect coverage changes

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[sy-cui]

LGTM

[armantekinalp]

LGTM. As a next step lets refactor examples that uses contact functions and friction and do a PR right after this.

[skim0119]

LGTM 👍