Feature request: paper variable for odd-only or even-only pagination

[Simon Albrecht]

Hello everybody,

this is a suggestion by Urs from a user list thread on 2018-08-30: for 
putting together scores of four-hand music with the typical primo right 
page, secondo left page layout it would be useful to select through a 
paper variable whether the music of a \book be typeset on only odd or 
only even pages, with according page numbers and margins (in case of 
twosided = ##t).

Best, Simon