[ms, c-c] new command checking for consistency / potentian flaws and enhancements of mapping file of a multi-sources instance

[repository] enabled sources refactoring: to avoid error, we should always have all known source in repo.sources_by_uri and only enabled ones in repo.sources, so we still have access to the definition of temporarily disabled sources

[migration] enhance RqlForIterator so we can do 'for e in rqliter(...).entities()'

[ms] when entity is coming from an external source, don't add non final relations on complete

[querier] add_types_restriction_cleanups: remove useless try/except (we already filtered out variables not in newroot.defined_vars) and dict accss. Also systematically set computed solutions instead of modifying existing ones

improvements in dataimport.py * updated the docstrings * fixed small issues in ucsvreader_pb * help user in using RQLObjectStore

add missing semi colon in js

[book] some more documentation and cleanups * merged existing facets documentation with the one I've just written and put almost everything within the code * added a note about __depends__ and __recommends__ and about the recommends semantic (extracted from a post on the ml) * added a note about write security checking (extracted from a post on the ml) * fixed some dumb sphinx errors

new RelationAttributeFacet + other fixes and cleanup + facets test * added documentation to Relation[Attribute]Facet and AttributeFacet explaining how to configure them * fixed behaviour of sorting for various combinaison of attribute values

[migration] remove bad test when constructing attributes to copy in rename_entity_type: the old entity type is not in the fschema due to schema deserialization trick

cleanup

[migration] fix rename_entity_type: avoid removal of attributes on the new entity type from the schema during migration

[migration] we can now specify attributes to backport to rename_entity_type using its attrs argument

[formwidgets] new allow_none attribute on BooleanField allowing a third None value for booleans

[primary view] consider rtypevid flag for relations in the 'relations' section as well