page: center and spread: none

[qnga]

page: center has been more or less deprecated in EPUB for interesting reasons that apply to RWPM as well.

The discussion pointed out that page: center was more about the lack of spread than centering because all spreads are centered and fitted. In RWPM too I guess we use it mainly to convey the idea that the page must not be part of a spread, which is quite confusing. I think it would be better to have a better named property and to have a discussion about it at the same time as we think about #103.