Add Modal transformations (issue4126042)

[tdanielsmusic]

Reviewers: ,

Message:
The set of modal transforms provided by Mike Ellis are now ready for
review.  As the changes are all in Scheme code they are easy to test.

Patch set 1 are as originally provided by Mike; patch set 2 are after
some tidying up by me.

As yet there is no documentation.  I'll begin writing that now, but I'd
prefer to see comments on the code before I get too far into that.

Trevor

Description:
Add Modal transformations

 - as provided by Michael Ellis <address@hidden>

Please review this at http://codereview.appspot.com/4126042/

Affected files:
  A input/regression/modal-transforms.ly
  M ly/music-functions-init.ly
  M scm/lily.scm
  A scm/modal-transforms.scm