Major commit: 
- change of the implementation of Pdb (interface remains backwards compatible). Now data stored in a proper object-oriented way. Pdb not abstract anymore, a protein model can be created by using one of the constructors
- added many tests and test files for Pdb (and merged some already existing tests from the default package)
- removed obsolete classes related to msd
- bug fix: PdbfilePdb now properly reads files with alt codes
- all tests passed