Added support for calculating center of pressure and force for Type-2 force plates. Type-3 force plates are still not functional.
Tests are also broken due to adding required parameters on parsing. Some (most) C3D files exclude required parameters such as the ANALOG group when they don't apply to the specific file. This is against the C3D specification. As a result, c3dio adds the missing parameters rather than rejecting the file. Upon writing, the number of parameters becomes different between the file and the read-write-read test fails.