Don't require opencv-python if opencv is already installed (#110)

This will allow us to get rid of py-opencv-python in our Spack
deployment

Author: heerener

setup.py

@@ -25,7 +25,6 @@
     "dataclasses; python_version < '3.7'",
     "matplotlib",
     "numpy",
-    "opencv-python",
     "requests",
     "scikit-image",
 ]
@@ -46,6 +45,12 @@
     "docs": ["sphinx>=1.3", "sphinx-bluebrain-theme"],
 }
 
+try:
+    # Could already be installed on the system
+    import cv2  # noqa
+except ImportError:
+    install_requires.append("opencv-python")
+
 description = "Search, download, and prepare atlas data."
 long_description = """
 Among different sources of data, Allen Brain Institute