Issue 2403: MultiMeasureRest's 'staff-position in RhythmicStaff is wrong per default

Issue 2403: MultiMeasureRest's 'staff-position in RhythmicStaff is wrong per default

The multimeasure rest does not look good for a staff line count of 2
(one would need to extend the test in this patch to check for line 1
in a similar way as line 2 and 3 are currently checked), but then it
does not seem worth bothering as long as the normal whole rest looks
awful for a staff line count of 2 as well.

Test with
\version "2.15.31"

\new StaffGroup {
$(make-simultaneous-music
   (map
      (lambda (x)
         #{ \new Staff \with { \override StaffSymbol #'line-count = #x } {
	       c'1
	       r1
	       R1
	       \override MultiMeasureRest #'staff-position = #4
	       R1
	     }
         #})
   (iota 6)))
}