Move Acceleration struct into its own file

[naterpotatoers]

I've noticed we keep recreating this struct since they its usually defined within a class themselves. This seems a bit silly since we have to then recreate the struct if we want to use it in another class.

By making this struct be fully independent of a class in their own file we can more easily update and manage the values in one centralized location reducing duplication. Not sure if this is a good practice but it makes sense to me moving forward

[naterpotatoers]

Also this would clean up the unit tests since we have to redefine the structs there too usually... Maybe we could move all structs within their own files for clarity

[Coreyboy1820]

I don't see why not. However I just fixed this issue, I thought I removed all instances of acceleration other than in the joint class and I removed this instance of it wherever it was.

[naterpotatoers]

okay alternative solution was found