Deprecating spatialmath.py and replacing with spatial-casadi

[cmower]

Description

This PR provides a fix for #113.

This PR will completely deprecate spatialmath.py and replace it with spatial-casadi. Spatial casadi is a cleaner interface, and is also unit-tested. The models.py script is becoming very difficult to maintain, due to having so many methods.

There is quite a big overhaul and many methods from the RobotModel class will be deprecated. However, no functionality will be lost and it is hoped that this update will clean up and modernise the main optas interfaces.

@joaomoura24, no need to look over this just yet - it is still early stages, and I have to ensure all the unit tests pass.

Todo

• [ ] Completed implementation of new feature.
• [ ] Add unit-test for the new feature.
• [ ] Add documentation for the new feature.
• [ ] Ensure all unit tests pass.
• [ ] Address all issues raised by unit-tests.
• [x] ~Consider adding an example that demonstrates the use of the new feature (not always necessary).~
• [x] Added code reviewer.

[cmower]

This is going to take some work to rebase. Probably it is best to restart this from scratch.