Add import check for shapely/geos (#110)

Closes #109

* add import check to __init__.py

* add commentary to installation docs

* spelling error

docs/sphinx/installation/index.rst

@@ -15,6 +15,17 @@ The easiest way to install ``pvfactors`` is using pip_:
 
    $ pip install pvfactors
 
+However, installing ``shapely`` from PyPI may not install all the necessary binary dependencies.
+If you run into an error like ``OSError: [WinError 126] The specified module could not be found``,
+try installing conda from conda-forge with:
+
+.. code-block:: shell
+
+   $ conda install -c conda-forge shapely
+
+Windows users may also be able to resolve the issue by installing wheels from `Christoph Gohlke`_.
+
+.. _Christoph Gohlke: https://www.lfd.uci.edu/~gohlke/pythonlibs/#shapely
 
 
 pvlib implementation

pvfactors/__init__.py

@@ -4,6 +4,22 @@
 import logging
 logging.basicConfig()
 
+try:
+    from shapely.geos import lgeos
+except OSError as err:
+    # https://github.com/SunPower/pvfactors/issues/109
+    msg = (
+        "pvfactors encountered an error when importing the shapely package. "
+        "This often happens when a binary dependency is missing because "
+        "shapely was installed from PyPI using pip. Try reinstalling shapely "
+        "from another source like conda-forge with "
+        "`conda install -c conda-forge shapely`, or alternatively from "
+        "Christoph Gohlke's website if you're on Windows: "
+        "https://www.lfd.uci.edu/~gohlke/pythonlibs/#shapely"
+    )
+    err.strerror += "; " + msg
+    raise err
+
 
 class PVFactorsError(Exception):
     pass