Add interpretation for default stem direction of center-line notes

[craigsapp]

Create an interpretation to control the stem direction of notes on the center line of a staff (primary for unbeamed notes, but could be adapted to beamed notes).

Example default behavior is the modern style of stem down:

**kern
*M4/4
*clefC3
=1
2B
2c
2d
*-

Setting the default position to stem up, where *mida means "middle-line notes have stems above":

**kern
*M4/4
*clefC3
*mida
=1
2B
2c
2d
*-

*midb would cancel (meaning "middle-line notes have stems below"). Or it could be *Xmida which might be nice for more consistent cancellation related to other interpretations such as *Xped:

**kern
*M4/4
*clefC3
*mida
=1
2B
2c
2d
*Xmida
2B
2c
2d
=
*-

This will allow better clef changes and transpositions by avoiding a fixed stem direction on a note that moves off of the center line (and for notes that move onto the center line).