layout blocks for book and bookpart

[Valentin Petzel]

Hello,

while paper blocks can be specified in books and bookparts, layout blocks 
cannot. This makes having different default layouts for multiple books or 
bookparts quite a hassle, as it requires specifying it for each score. Also 
this makes having different stylesheets for different bookparts quite 
impossible.

So I want to ask if there is any reason against allowing layout blocks in books 
and bookparts, having them override the default layout for that book or 
bookpart.

If not, would you consider it reasonable to do it this way.

Best,
Valentin