a midifile can be inadvertently written with absolute ticks

[belm0]

Currently it's possible to call make_ticks_abs() on a Pattern and write it out to a midifile with absolute ticks, which doesn't adhere to the MIDI file standard. (aplaymidi and some other MIDI tools seem to support this non-standard tick format, which can make more of a problem since you won't realize you're creating bad files until you use other tools.)

write_midifile() should at least assert that the pattern's tick mode is relative.