add challenging test files for combining

[orbeckst]

We need test files that allow us to develop better "combining" functionality.

• 3 trajectories
• each should have (1) same protein, (2) different number of the same lipid type
• trajectory: xtc less than ~2 MB each
• topology file (PDB or GRO) for each
• strip water and other lipids

(E.g. CG prestin + POPG or POPC)

Add the files to the data directory and include variables in datafiles.py.

[orbeckst]

Compress PDB/GRO files with bzip2 (and add the xxx.pdb.bz2 files).