implement simplified solis clear sky model (#148)

* implement simplified solis. needs tests

* fix index stripping

* add tests. coerce pw to min 0.2

* add simplified solis option to location.get_clearsky. reorder simplified solis output to match ineichen

* update whatsnew

* remove return_raw in favor of OrderedDict. improve nan handling

Author: wholmgren

docs/sphinx/source/whatsnew/v0.3.3.txt

@@ -1,6 +1,6 @@
 .. _whatsnew_0330:
 
-v0.3.3 (June xx, 2016)
+v0.3.3 (June 15, 2016)
 -----------------------
 
 This is a minor release from 0.3.2.
@@ -32,6 +32,7 @@ Enhancements
   (:issue:`190`)
 * Improve speed of ``singlediode`` function by using ``v_from_i`` to
   determine ``v_oc``. Speed is ~2x faster. (:issue:`190`)
+* Adds the Simplified Solis clear sky model. (:issue:`148`)
 
 
 Bug fixes