add a filetype check BEFORE loading any files

[wkumler]

I keep getting burned by non-mzML files included in the pass to grabMSdata, but I only find out about it after I've sunk a bunch of time into extracting all the actual mzMLs first until it hits one and crashes. Two solutions for later: one, check the filetype before loading any of the files which is simple enough to do by checking the extension but might throw some false positives; or two, throw a warning that the file type isn't recognized and ignore it. Adding to the v1.4 milestone maybe?

[wkumler]

Completed with #24