Fix Python bindings installation and support building bindings as standalone projects

[traversaro]

I initially just fixed the installation of __init__.py for wearables and fixed compilation on Windows, to permit to merge robotology/robotology-superbuild#1717 (see 5e0ac8c, 72f02ae and 7b42767).

However, while I was at it, I thought it was useful to also fix robotology/robotology-superbuild#1268, propagating what was done in robotology/idyntree#1209 (5ed3e38).

Then, as this was necessary to package easily hde on conda-forge, I also added support to compile the python bindings as a standalone project, linking against an existing external HDE installation (5b6d09f and 002b75d).

This PR change the internal ALIAS of WearableData and WearableActuators target to WearableData::WearableData and WearableActuators::WearableActuators for consistency with the namespace used during the install. If you have been using wearables via FetchContent/add_subdirectory, please change all your instances of Wearable::WearableData to WearableData::WearableData and of of Wearable::WearableActuators to WearableActuators::WearableActuators.

[dariosortino]

Consider this approval as an ack of these modifications, I'm not very familiar with cmake and bindings in general

[traversaro]

Consider this approval as an ack of these modifications, I'm not very familiar with cmake and bindings in general

Yes, I think for most ami-iit's repos reviews are mostly an ack, unless the maintainer wants to go into deep and comments.