Run ModelChain from POA irradiance (#943)

* add _from_poa, _from_effective_irradiance methods

* indentation, lint

* sort out weather, total_irrad, effective_irradiance

* fix _prep_inputs_solar_pos

* press_temp

* reorganize code that assigns weather and total_irrad

* add prepare_temperature

* start fixing tests

* start on tests

* split run_model_from_effective_irradiance

* fix tests, sticker

* more test fixes

* another test fix

* add test for prepare_temperature

* don't replace missing poa_global with effective_irradiance

* make _prepare_temperature private

* add _assign_weather() to test

* method name to sapm_temp

* use the correct class

* api, whatsnew

* docstring improvements, use assert_frame_equal

* delete [k]

* add issue to whatsnew

Author: cwhanse

docs/sphinx/source/api.rst

@@ -547,14 +547,25 @@ Creating a ModelChain object.
 Running
 -------
 
-Running a ModelChain.
+A ModelChain can be run from a number of starting points, depending on the
+input data available.
 
 .. autosummary::
    :toctree: generated/
 
    modelchain.ModelChain.run_model
+   modelchain.ModelChain.run_model_from_poa
+   modelchain.ModelChain.run_model_from_effective_irradiance
+
+Functions to assist with setting up ModelChains to run
+
+.. autosummary::
+   :toctree: generated/
+
    modelchain.ModelChain.complete_irradiance
    modelchain.ModelChain.prepare_inputs
+   modelchain.ModelChain.prepare_inputs_from_poa
+
 
 Attributes
 ----------

docs/sphinx/source/whatsnew/v0.8.0.rst

@@ -105,6 +105,11 @@ Enhancements
 * Add :py:func:`pvlib.pvsystem.combine_loss_factors` as general purpose
   function to combine loss factors with a common index.
   Partialy addresses :issue:`988`. Contributed by Brock Taute :ghuser:`btaute`
+* Add capability to run a ModelChain starting with plane-of-array or effective
+  irradiance, or with back-of-module or cell temperature data. New methods are
+  :py:meth:`pvlib.modelchain.ModelChain.run_model_from_poa`,
+  :py:meth:`pvlib.modelchain.ModelChain.run_model_from_effective_irradiance`,
+  and :py:meth:`pvlib.modelchain.ModelChain.prepare_inputs_from_poa` (:issue:`536`, :pull:`943`)
 
 Bug fixes
 ~~~~~~~~~