Renamed a bunch of cached_robot to cached_robot_model in PyBulletClient

[yck011522]

The existing naming convention calling the cached robot_model as robot is very confusing. I renamed it to explicitly say cached_robot_model.

This is a breaking change for people who directly access the cached model, but I doubt anyone is doing that now.

• PybulletClient.get_cached_robot to PybulletClient.get_cached_robot_model
• PybulletClient.ensure_cached_robot to PybulletClient.ensure_cached_robot_model.
• PybulletClient.ensure_cached_robot_geometry to PybulletClient.ensure_cached_robot_model_geometry.
• PybulletClient.cache_robot to PybulletClient.cache_robot_model.

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
• I added new functions/classes and made them available on a second-level import, e.g. compas_fab.robots.CollisionMesh.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[gonzalocasas]

LGTM