convert_frame_cont_to_array does not work all the time

Sometimes, the keys in frame containers are not actually integers and the code fails in https://gitlab.idiap.ch/bob/bob.pad.base/blob/172d46e0ca734ce682323c502f23abc53da15f4b/bob/pad/base/utils/helper_functions.py#L35 when keys are converted from strings to integers.