Added support for Desmond Trajectory family.

First attempt to support Desmond trajectory family. When I try to run nosetests for the whole test suite, I ran into a weird issue with below message:
Fatal Python error: PyThreadState_Get: no current thread
Abort trap: 6
Probably due to some messy dynamic linking issues. In any cases, I am trying my best to test the unit test for dtr module, and it does pass the test.
what's the reason for importing the same struct from two different c header files? (e.g. here and line 10). I think one of them is just going to overwrite the other... it's probably only necessary to declare the struct once.