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: 254bd456f49d61933fa27dec6802360e26d31235 [formerly a3532e8ecff9ade6a4f6a82e7dbc5cc68c0be0c2] [formerly a3532e8ecff9ade6a4f6a82e7dbc5cc68c0be0c2 [formerly 93e2bcbfa09ac378ff34e04777c0983329017d53]]
Former-commit-id: 61d57a8f53e4fd8706b6e9bc543ac58d18501405
Former-commit-id: ca32468797e10b1bd43728a0934cf0ffbcc6bc57
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine