Separate wrapper from the main repository, include it as submodule

[heuristicus]

Prototype to have a unified interface to the SDK which can be used by both the ROS and ROS2 versions. See #113 for more details about that. The wrapper repository can be found at https://github.com/bdaiinstitute/spot_wrapper. See bdaiinstitute/spot_wrapper#1 for details about things which still need to be worked out.

This removes the wrapper from the repository and includes it as a pure python submodule instead. This requires some small modifications to how the repository is installed.

When cloning, it's necessary to run

git submodule init
git submodule update

to make sure the wrapper is pulled in correctly.

It's also necessary to install the wrapper with

pip3 install -e spot_wrapper

This will install the wrapper which should then be available to when running python. The -e flag means that it's installed such that any changes to the files carry over to the installation immediately. Without this it's necessary to install the wrapper every time files change.

Other than this initialisation change, the wrapper should behave as before, but this needs some testing as the wrapper repository is a combination of the ROS1 and ROS2 versions of the wrapper, which had some differences.