Nzshm-model

Changed
- `NhsmModel` initialized with component objects (i.e. LogicTree objects) rather than files
- Creating `NshmModel` object from files handled by classmethod `from_files`

Removed
- `NshmModel.gmm_logic_tree_from_xml` method. Use `OpenquakeGMCMPshaAdapter.gmcm_logic_tree_from_xml` instead

Added
- support for sites in `OpenquakeConfig` and `OpenquakeConfigPshaAdapter`
- usage documentation for `OpenquakeConfig` and adapter classes

Changed
- flexibility in defining uniform site parameters

Added
- OpenQuake adapter classes for source logic tree, gmcm logic tree, and hazard configuration
- `OpenquakeSimplePshaAdapter` writes entire set of OpenQuake input files
- hazard_config attribute of `NshmModel` class
- OpenQuake configuration of hazard_config for official models "NSHM_v1.0.0" and "NHSM_v1.0.4"

Changed
- Models args generated by factory function for official models retrieved by `get_model_version()`

Removed
- `NshmModel.source_logic_tree_nrml()` method

Changed
- model versions defined as argument dictionary rather than a NshmModel instance
- model versions no longer loaded as modules by get_model_version()

Removed
- slt_config attribue of NshmModel

Added
- easy to read string representation of LogicTree
- BranchSet class is an iterator which can be used to access its branches

Changed
- elevate stable version2 of source logic tree modules to top of source_logic_tree package
- optional dependencies for minimum install