Axis_group_engraver + Scheme engraver for staff combining (issue 576540043 by address@hidden)

[pkxgnugitcl]

Reviewers: ,

Message:
Passes make, make check and a full make doc.

Description:
Axis_group_engraver + Scheme engraver for staff combining

It provides an intuitive user interface
to control whether shared or separate
staves are printed, and should work
for combinations of 4 or more parts.

Currently, Axis_group_engraver reads
keepAliveInterfaces into an internal
variable during process-music, then
uses the stored value during
acknowledge-grob.

In this patched it waits to read
keepAliveInterfaces during
acknowledge-grob.

Please review this at https://codereview.appspot.com/576540043/

Affected files (+128, -12 lines):
  M lily/axis-group-engraver.cc
  A ly/divisi_engraver.ly