Add description for MEI virtual unit

[doerners]

This PR adds a description of the nature of MEI virtual unit to chapter 3 of the Guidelines.
Closes #782

[rettinghaus]

Looks good to me, thanks!

[musicEnfanthen]

Thank you @doerners .

Please note that there is also

music-encoding/source/modules/MEI.visual.xml

Lines 1493 to 1496 in 374ce48

	<attDef ident="vu.height" usage="opt">
	<desc>Defines the height of a "virtual unit" (vu) in terms of real-world units. A single vu
	is half the distance between the vertical center point of a staff line and that of an
	adjacent staff line.</desc>

Maybe it's possible to harmonize the wording in all three passages to avoid diverging interpretations?

[bwbohl]

In terms of harmonisation I'd prefer the "middle" of the staffline, because then page orientation is not of importance.
If you would be so kind as to harmonise it in the other spots (visual and shared)

[bwbohl]

2022-02-24 ODD meeting: As soon as it's done and reviewed we can merge it.

[musicEnfanthen]

Looks good. Thank you @doerners