removed cache dir, cache is now stored in memory as self.output
name is not obligatory. By default use transformer.__class__.__name__
name by default is added suffix '_0' to the name (or 1,2,3,...)
now you can clean_cache or clean_cache_upstream_steps
added name validation
new defaults:
is_fittable=True
force_fitting=True
cache_output=False
persist_output=True