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Enhancements
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Added module spacing and transparency inputs to irradiance models, and updated README file to make it clearer.
* Add module transparency and spacing to irradiance models (72)
* Use reStructuredText for README and add TOC (74)
Fixes
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Fix small issue in irradiance models for fast mode: made sure that shaded surfaces are not getting any Perez circumsolar irradiance, except via module spacing and transparency.
* Fix irradiance models for fast mode shaded surfaces (73)
Contributors
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* Marc Anoma ([anomam](https://github.com/anomam))