Pvfactors

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Requirements
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* Add python 3.9 to test configuration (122)
* Set the upper bound on shapely to version 2.0 (not yet released). The shapely dependency may be dropped altogether in a future pvfactors release. (130 )


Fixes
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* A small bug in the pvlib-python implementation of the Perez transposition model was discovered and fixed in pvlib v0.9.0. To ensure the error does not affect pvfactors output moving forward, the pvlib dependency is updated from ``pvlib>=0.7.0,<0.9.0`` to ``pvlib>=0.9.0,<0.10.0``. This will likely change the results of irradiance simulations. According to the [pvlib release notes](https://pvlib-python.readthedocs.io/en/v0.9.0/whatsnew.html#bug-fixes), the differences are "expected to be small and primarily occur at low irradiance conditions". (121)
* Fixed a bug that affected some irradiance simulations when `surface_tilt` is exactly zero. See 125 for details. (128)

Maintenance
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* Update CI including sphinx for documentation (124)
* Add documentation for making new releases (133)

Contributors
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* Marc Anoma ([anomam](https://github.com/anomam))
* Kevin Anderson ([kanderso-nrel](https://github.com/kanderso-nrel))
* Mark Campanelli ([campanelli-sunpower](https://github.com/campanelli-sunpower))

=======================

Enhancements
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* Update pvlib dependency from ``pvlib>=0.6.0,<0.8.0`` to ``pvlib>=0.7.0,<0.9.0`` (116)


Contributors
------------
* Marc Anoma ([anomam](https://github.com/anomam))
* Kevin Anderson ([kanderso-nrel](https://github.com/kanderso-nrel))

==========================

Enhancements
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* Add import check for shapely/geos (110)
* Drop Python 2.7, 3.5, add Python 3.8 (112)


Fix
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* TsSegment was missing proper indexing (102)
* Fix CI: restrict pvlib to <0.8.0 because of API break, reduce test length because of hanging CI (112)

Contributors
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* Thomas Capelle ([tcapelle](https://github.com/tcapelle))
* Kevin Anderson ([kanderso-nrel](https://github.com/kanderso-nrel))
* Marc Anoma ([anomam](https://github.com/anomam))

==========================

Fix
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The vectorization of the calculations (from v1.3.0) in the PVEngine had removed the ability to account for timeseries albedo values (it was only using the first albedo value). This fix repairs that issue by building the full 3D matrices for the reflectivity values (and the inverse reflectivity values as well).

* PVEngine needs to use timeseries albedo values (98)

Contributors
------------

* Marc Anoma ([anomam](https://github.com/anomam))

==========================

Enhancements
------------

pvfactors can now account for AOI losses by either using constant diffuse losses, are by using an fAOI function that will provide the corresponding loss for each value of angle of incidence.

* Test for continuity of results with direct shading (91)
* Implement non-diffuse AOI loss methods (92)
* Implement fAOI modifiers for irradiance models (93)
* Merge new AOI methods into full mode workflow (94)
* Include fAOI losses from irradiance models in tests (95)
* Update docs for AOI methods (96)


Contributors
------------

* Marc Anoma ([anomam](https://github.com/anomam))

=========================

Enhancements
------------

pvfactors is now only using timeseries geometries and vectorization for the view factor matrix calculation, even with the full reflection equilibrium mode.
This resulted in an incredible speed boost, in which 8760 simulations now run in less than 2 seconds when using the full mode (it previously took a couple minutes). So there's not much reason anymore to use the "fast" mode, which is less accurate and not that faster anymore.
Lots of package clean up and documentation updates in addition to this.

* Create timeseries ground elements (80)
* Index all timeseries surfaces (82)
* Vectorize calculation of vf matrix (83)
* Implement vectorized full mode (84)
* Clean up package now that full mode is vectorized (86)
* Reorganize geometry sub-package (87)
* Add docs section on main concepts (88)
* Update docs tutorials (89)


Contributors
------------

* Marc Anoma ([anomam](https://github.com/anomam))