Sort files for Mac functionality

[priyankshyam]

When opening multiple sequentially listed files in debug mode (for ex. the .xyy test files), the generated 2D plots are have unusual striations and the files are loaded non-sequentially. Probably needs a sort function in the openTestfiles method:

def openTestfiles(self):  
        path = os.path.abspath('_test_files')
        test_sets = os.listdir(path)
        for tset in test_sets:
            ts_path = os.path.join(path,tset)
            files = [os.path.join(ts_path,f) for f in os.listdir(ts_path) if f.endswith(tset)]
            ext = '*.'+files[0].split('.')[-1]
            self.addDataset(self,files=files,ext=ext)