add converters to cndbTools

[s-brahmachari]

the converters should have flags to identify a snippet of the trajectory. Given a trajectory of say 1M frames in cndb, one should be able to use the converters to create a .gro/.ndb/.pdb trajectory containing say, 1000 frames starting at t1 and ending at t2, or just one frame.

[contessoto]

Just adding that we need to add the newer version of cndb into the code. -When loading a cndb file, it must check the file version; -Need to add the converter/encoder from .ndb to cndb.