Move MpdPhysics library to dedicated folder

Currently physics/CMakeLists.txt is used both to build MpdPhysics library as well as to load (and build) other physics connected libraries which is bad from the point of view of decoupling of CMakeLists.txt concerns. To solve this problem, build of MpdPhysics library will be moved to the dedicated subfolder physics/mpdPhysics and the original physics/CMakeLists.txt will be used exclusively to build all physics-related libraries.

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