Fixes and tests corrected

Changed

• The step function execution speed has been increased by 25% when return_res is True! Small performance improvement when return_res is False.
• The size argument of step function is now known as width.
• We now require pylinkage>=0.4.0.

Fixed

• Files in leggedsnake/examples/ were not included in the PyPi package.
• The example was incompatible with pylinkage 0.4.0.
• Test suite was unusable by tox.
• Tests fixed.

Security

• Tests with `tox.ini`` now include Python 3.9 and Flake 8.