logictree: cosmetic changes according to review comments
logictree: cosmetic changes according to review comments

* no more cache in Branchset._is_somejavaclass() methods
* a small change in Branchset's class docstring
* changed comment in Branchset.sample() for clarity


Former-commit-id: 93e2bcbfa09ac378ff34e04777c0983329017d53
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine