Implement 'hookgroup' section parsing

[ulteq]

Currently unsupported in the new parser: https://github.com/RigsOfRods/rigs-of-rods/blob/86b6bc5f95b9abd937b270747b2c7e0137b65fe8/source/rig_file_input_output/ReadMe.txt#L127

Reference (old parser): https://github.com/only-a-ptr/ror-legacy-svn-trunk/blob/0a55a993899d6a06cdc58fd98b5b675b371eb89a/source/main/physics/input_output/SerializedRig.cpp#L4488-L4507

This code is unreachable without hookgroup sections: https://github.com/RigsOfRods/rigs-of-rods/blob/72cf78054dc3e11ab772c19dc1267e177daf09aa/source/main/physics/Beam.cpp#L4429-L4446

This does not affect normal hooks or lockgroups sections.

[ohlidalp]

I just noticed hookgroup section support is missing in upstream, and also in docs. This means we can't create hooks which only lock ropables, although the update code is still in place.

How it worked in 0.3x:

• section def: https://github.com/only-a-ptr/ror-legacy-svn-trunk/blob/0a55a993899d6a06cdc58fd98b5b675b371eb89a/source/main/physics/input_output/SerializedRig.cpp#L107
• parsing: https://github.com/only-a-ptr/ror-legacy-svn-trunk/blob/0a55a993899d6a06cdc58fd98b5b675b371eb89a/source/main/physics/input_output/SerializedRig.cpp#L4488-L4505