Clearer name for time attribute

[page200]

The attribute name time doesn't explain whether it is for time since the beginning of the song or since the previous MIDI message, nor whether it is given in seconds or milliseconds or ticks.

Moreover, the units (ticks vs. seconds) are inconsistent, e.g. print(mid) and print([*mid]) shows different time values, despite the attribute being called time in both cases.

Instead, there can be two attributes: delta_ticks and delta_seconds. "Delta" in the name indicates that these are the ticks/seconds since the previous MIDI message, rather than since the beginning of the song.

Converting between the two requires knowing the previous tempo-change message. That might be the reason why the time in seconds is computed only during a loop through all messages (i.e. during __iter__()) and then discarded. One option is to keep it that way. (An alternative is to keep track of tempo changes and let the setters of delta_ticks and delta_seconds set both attributes at once. As we look into keeping track of pitch-bend range changes in issue #514, insights about keeping track of tempo changes might emerge.)

[page200]

delta_seconds should use fractions, see issue #528.