Equivariant Feature Extraction Example

[djoca77]

Fixes # .

Description

A few sentences describing the changes proposed in this pull request.

This adds an example of an equivariant feature extraction on a 2D slice of 3D .nii.gz brain MRI image. This change adds a new optional dependency (e3nn) along with adding a testing image. For the time being, the testing image has been added to testing_data while the process of having it added to MONAI-extra-test-data happens

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• [ x] Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• [ x] In-line docstrings updated.
• [ x] Documentation updated, tested make html command in the docs/ folder.