Avoid side effects in mdtraj.load
Avoid side effects in mdtraj.load

using `filename_or_filenames.pop(0)` leads to a change of the filename list which propagates back into the outer scope and is (in my opinion) quite unexpected, i.e., handing in a list of filenames, that list is suddenly shorter by 1 after a call to `mdtraj.load`. This small change uses slicing rather than `pop` which as an added benefit also supports tuples (which do not possess `pop`).
rmcgibbo/mdtraj,mdtraj/mdtraj,gph82/mdtraj,gph82/mdtraj,rmcgibbo/mdtraj,gph82/mdtraj,rmcgibbo/mdtraj,mdtraj/mdtraj,mdtraj/mdtraj