Make written MIDI tempo rounding opt-in - #266

Addresses #103 with opt-in rounding while preserving existing writes.

PrettyMIDI.write(filename, round_tempo=True) uses mido.bpm2tempo() to round MIDI tempos to the nearest integer microsecond per quarter note. For example, 123 BPM becomes 487805 instead of 487804. Omitting the argument, or passing round_tempo=False, retains the previous truncation behavior.

The method documents the option. Regression tests decode written MIDI bytes and cover the default and explicit modes, integer tempo changes across four resolutions and three read/write cycles when rounding is enabled, boundary tempo values, unchanged note ticks, and unchanged in-memory timing data.

Validation on Linux with Python 3.12, NumPy 2.5.4 and mido 1.3.3:

• python -m pytest tests -q: 29 passed, with the existing adjust_times warning.
• Restoring the previous PR source makes all four tempo tests fail: the compatibility test detects the changed default, and the three rounding tests detect the missing option.
• A separate 32-case comparison of complete serialized MIDI files matches upstream bytes for both the omitted and explicit-false modes, and matches the previous rounded PR bytes for the explicit-true mode. Each case includes tempo changes and notes.
• python -m flake8 tests/test_write_tempo.py and git diff --check: passed.

The legacy nosetests -v --with-coverage --cover-package=pretty_midi command cannot start on Python 3.12 because nose imports the removed imp module. Python 2.7/3.5 and DAW playback were not tested.