[spaces.cg.base] update to the 21st century^^
[spaces.constraints] added ConstraintsInterface

* put all in Spaces namespace
* renamed LocalDefault -> internal::Default (not intended for end-users)
[spaces.interface] updated local_constraints()
[assemlber.wrapper] updadet constraints usage
[spaces.cg.base] updated constraints usage
[test.elliptic-cg-discretizations] updated constraints