Fix precentering overflow error in center.c (#1283)
Fix precentering overflow error in center.c (#1283)

* Fix precentering overflow error in center.c

* Change size_t to long long, since OpenMP on windows doesn't like unsigned values for loop variables.

leeping/mdtraj,mattwthompson/mdtraj,gph82/mdtraj,mdtraj/mdtraj,gph82/mdtraj,jchodera/mdtraj,jchodera/mdtraj,rmcgibbo/mdtraj,leeping/mdtraj,mattwthompson/mdtraj,jchodera/mdtraj,rmcgibbo/mdtraj,mdtraj/mdtraj,gph82/mdtraj,mdtraj/mdtraj,mattwthompson/mdtraj,dwhswenson/mdtraj,mattwthompson/mdtraj,rmcgibbo/mdtraj,leeping/mdtraj,dwhswenson/mdtraj,dwhswenson/mdtraj,jchodera/mdtraj,leeping/mdtraj