[core.interfaces] bugfix
[analyticalproblems] make problems immutable
[analyticalproblems] fix sid creation in BurgersProblem
[core.interfaces] fail silently if sid calculatin fails in inject_sid
[core.cache] add clear() to CacheRegion
[core.cache] add clear_caches()
[core.cache] make filename configurable for DogpileDiskCacheRegion
[discretizers.advection] small tweak to sid calculation
[core.cache] revamp code for globally disabling caching
[core.cache] allow to disable caching per region