GSIM logic tree

[chrisdicaprio]

We want to support ground motion logic trees in the same way that we do with SRM logic trees. Done when:

• serializable GMCMLogicTree class that describes GMCM logic tree including branch sets for each TRT
• Identify TRT
• #36 (moved to seperate ticket, not required to close)
• GMCMBranch class identifies gsim class and parameters used by OpenQuake
• can be written to xml using OpenquakeSimplePshaAdapeter method
• can be read from json
• can be read from OpenQuake nrml
• test coverage

Notes:

• OpenquakeSimplePshaAdapeter.__init__() has both source_logic_tree and gmsm_logic_tree args that are optional

Extras:

• rename FaultSystemLogicTree class