Add import check for shapely/geos

[kandersolar]

Closes #109

One consequence of doing this check in __init__.py is that it makes it impossible to use pvfactors at all if shapely.geos.lgeos can't be imported. I've not actually used pvfactors before so maybe this is a silly question, but is there any pvfactors functionality that doesn't require shapely? If so, maybe this warrants a more targeted change.

I ran into difficulties testing py2.7, but here's the output I get with py3.7:

(pvfactors-dev) C:\Users\KANDERSO\projects\pvfactors>python -c "import pvfactors"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\KANDERSO\projects\pvfactors\pvfactors\__init__.py", line 21, in <module>
    raise err
  File "C:\Users\KANDERSO\projects\pvfactors\pvfactors\__init__.py", line 8, in <module>
    from shapely.geos import lgeos
  File "C:\Users\KANDERSO\Software\Anaconda3\envs\pvfactors-dev\lib\site-packages\shapely\geos.py", line 154, in <module>
    _lgeos = CDLL(os.path.join(sys.prefix, 'Library', 'bin', 'geos_c.dll'))
  File "C:\Users\KANDERSO\Software\Anaconda3\envs\pvfactors-dev\lib\ctypes\__init__.py", line 364, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: [WinError 126] The specified module could not be found; 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 Cristoph Gohlke's website if you're on Windows: https://www.lfd.uci.edu/~gohlke/pythonlibs/#shapely

I'm happy to push updates if this needs anything else like a changelog entry or something, let me know.

[anomam]

Thanks a lot @kanderso-nrel, this is the right approach in my opinion. shapely is currently marked as a requirement for the whole package so we shouldn't allow partial usage of pvfactors and do a module-level warning, it would be quite messy I think.

[anomam]

👍

[anomam]

thanks