Clean up OpenSim::PointForceDirection

[adamkewley]

Minor clean-up. The motivation for doing so is that I'm currently working on a new ForceEmitter base class, which can re-use PointForceDirection, rather than (e.g.) having to introduce its own point-force holding class.

Fixes issue N/A

Brief summary of changes

• Remove virtual destructor. It's unused codebase-wide
• Deprecate scale feature. It's unused in opensim-core, which uses the magnitude of direction to encode the force
• BREAKING: add a final qualifier to the class, which will cause all code that was inheriting PointForceDirection to no longer compile, which is a requirement for removing the virtual destructor
  • It's unlikely downstream code will be inheriting from this. It's a mostly-internal-use class that only pops up as a datastructure passed out by GeometryPath

Testing I've completed

• None: they're basic depecations

Looking for feedback on...

• Basics

CHANGELOG.md (choose one)

• Updated with a note about the final and [[deprecated]] changes. Unlikely to affect downstream users.

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This change is 

[adamkewley]

Do you think it's worth it to upgrade this to a Socket?

Not in this particular case: the class is a plain C++ class, rather than an OpenSim::Component, so it doesn't have (e.g.) the socket table that's used to store socket declarations - it's essentially an internal struct for the C++ API that seems to mostly just be used by GeometryPath

[adamkewley]

Thanks for the comments @nickbianco and @tkuchida - I've addressed them as best as possible, including rewriting some of the documentation.

Since this is a minor change, I'll go ahead and merge it, rather than doing multiple rounds