PitchClass should allow working with IntervalClass in transposeBy, interval methods

[albertms10]

music_notes/lib/src/note/pitch_class.dart

Lines 158 to 160 in 7ce5bee

	// TODO(albertms10): expect [IntervalClass]. See #248.
	PitchClass transposeBy(Interval interval) =>
	PitchClass(semitones + interval.semitones);

music_notes/lib/src/note/pitch_class.dart

Lines 171 to 173 in 7ce5bee

	// TODO(albertms10): return [IntervalClass]. See #248.
	Interval interval(PitchClass other) {
	final diff = difference(other);