Feature request: `.getOffsetBySiteInSeconds` in Music21Object

[oxygen-dioxide]

Motivation
We already have Music21Object.seconds that returns the duration of the object in seconds. I hope that we can have a function that returns the object's position relative to a stream it belongs to. It is useful in the following scenarios:

• Developing audio synthesizers that takes Music21 stream as input.
• Handling a stream with a lot of objects. A .getOffsetBySiteInSeconds will avoid cumulative error in summing the .seconds duration of all objects.

Feature summary
Add a method, .getOffsetBySiteInSeconds, that returns the position relative to a user-given stream it belongs to.

Proposed implementation

Optionally, describe the implementation or the interface, in code or pseudocode.

Like how .seconds is implemented

Intent

• I plan on implementing this myself.
• I am willing to pay to have this feature added.
• I am starting a discussion with the hope that community members will volunteer their time to create this. I understand that individuals work on features of interest to them and that this feature may never be implemented.

[mscuthbert]

Looks like a good idea. The name of the method should be .getSecondsBySite(Stream) as a parallel to .getOffsetBySite(Stream) -- I think we shouldn't confuse the concept of seconds and offsets.

If you wanted to find say the number of seconds of a note since the beginning of the Part/Score even when the note is in a measure or voice, etc. that would be .getSecondsInHierarchy(Score) as a parallel to .getOffsetInHierarchy.

But -- I'm also leaning towards going in a different direction and instead of adding these as methods to music21 object, we should put them in the tempo.py file as functions -- the project is now over 18 years old and this use case hasn't come up often at all in that time, so the (mental) overhead of learning these functions should probably only go on people who are looking for tempo tools and not everyone working with notes, etc.

(secondsMap on Stream might similarly become deprecated and moved to tempo.py also).

[oxygen-dioxide]

For why I come up with this name:
Because the existing function name seconds is confusing to me. When I saw it, I didn't know if it is duration in seconds or position (offset) in seconds. So I think we should keep more information in function name.

	time unit: quarter note	time unit: second
length (duration)	.quarterLength()	.seconds()
offset (position)	.offset()	(what we currently don't have)

[mscuthbert]

It might be a need for something else, but we are not going to reuse offset to mean anything more than it currently means.