Add sphere collision avoidance constraints

[cmower]

Description

Adds sphere collision avoidance helper constraint method. Also, a minor addition that adds reduce_constraint option for equality constraints.

Todo

• [x] Completed implementation of new feature.
• [x] Add documentation for the new feature.
• [x] add unit test.
• [x] Ensure all unit tests pass.
• [x] Address all issues raised by unit-tests.
• [x] Add example.