[docs] bugfix
[operators] add projected to OperatorInterface
[operators] add missing linear attributes
[operators] remove sparse attribute form MatrixBasedOperatorBase
[caching] remove __init__ from CacheableInterface

since mixins should not have __init__ methods if it can be
avoided. Otherwise using super in __init__ becomes unusable
[core] remove __init__ from BasicInterface
[caching] replace 'caching' by 'cache_region' everywhere
[tests] update caching tests
remove unnecessary __init__ calls and use super() everywhere else
[caching] forgot to change 'caching' to 'cache_region' here
[operators] bufix