Add support for SMPTE based SMF division

[rdoursenaud]

Standard MIDI Files (SMF) can specify their time base division in two formats:

• ticks per quarter-note (aka notation-based)
• negative SMPTE format + ticks per frame (aka time-based)

Currently, mido only supports the first format and creates invalid data when the second is present.

Spec

This information is stored in the Header Chunk of the SMF in the division field:

b'MThd'    # Header chunk magic (4 bytes)
<length>   # Length of the following data. At least 6 bytes. (1 byte)
<ff>       # Format type (2 bytes representing a 16-bit word MSB first aka big-endian)
<ntrks>    # Number of tracks (2 bytes representing a 16-bit word MSB first aka big-endian)
<division> # Time base division (2 bytes representing a 16-bit word MSB first aka big-endian)

The data is a 16-bit word (2 bytes big-endian).
The most significat bit represents the time base division format:

• 0: ticks-per quarter note.
  The value is stored in the 15 other bits in big-endian form.
• 1: SMPTE.
  Negative SMPTE format encodes the framerate onto the remaining 7 bits of the most significant byte.
  The least significant byte encodes the tick per frames.

Challenges

mido always expects ticks_per_beat (sic) for the moment.
A thorough review of the implications is required before implementation.
Including, but not limited to:

• Tempo determination
• Time units conversions
• Message time property