convert gmcm lt from oq psha adaptor to native type

[chrisdicaprio]

Currently NshmModel refers to the GMCM logic tree by the openquake xml and gmm_logic_tree property returns a psha_adapter.openquake.logic_tree.LogicTree. We want instead to use the nzhsm-model native GMCMLogicTree for the property and refer to the serialised object as json rather than OQ xml.

Done when

• NshmModel.gmm_logic_tree returns a GMCMLogicTree
• NshmModel GMCM member is the json (serialised) file rather than the openquake xml
• any backwards compatability with using the xml is preserved
• official versions 1.0.0 and 1.0.4 are converted to use the correct objects
• values and tag are moved up to the Branch base class
• official GMCM logic trees have populated values
• add API docs page and docstrings (similar to api/logic_tree/source_logic_tree.md )