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- Update documentation
- add new style PVPower demo model to examples
- add tests for both new and olds styles
- fix formula imports for packages

- fix 56 add model meta class to declare layers as class attributes
- fix 23 use commands from simulation registry
- removes BasicModel and combines with Model
- adds commnand layer model attribute to denote layer with commands
- adds registry model property, combines all registries into single dictionary
- state property now returns whether model is initialized

- fix 29 add tz to pvpower demo, localize timestamps
- test uncertainty propagation versus Uncertainties
- fix 52 scale covariance from calculation results
- update pvpower demo to pvlib-0.4
- fix 49 refactor layers to remove redundancy
- fix 26 use meaningful names in layers
- fix issue in formulas when retval units is None instead of [None]

- fixes 47 change permissions on scripts to executable
- fixes 48 use `install_requires` in `setup.py` to install dependencies

new readers in contrib
- `DjangoModelReader`
- `ArgumentReader`
- `HDF5Reader`

- fixes 38 let data sources accept any kind of argument, not just filename
- fixes 39 allow data caching to be disabled
- fixes 40 use Meta class for options
- fixes 41 pass data to `load_data()` not reader