fix installation error: declare cornet package

Before, find_packages looked only inside the `cornet` directory, declaring it as the root and making `import cornet` fail after a pip install.
This change sets the exported packages to only `cornet` and implicitly its children which remedies this issue.

setup.py

@@ -23,7 +23,7 @@
     author="Jonas Kubilius, Martin Schrimpf",
     author_email='[email protected], [email protected]',
     url='https://github.com/dicarlolab/CORnet',
-    packages=find_packages('cornet'),
+    packages=['cornet'],
     include_package_data=True,
     install_requires=requirements,
     license="GNU GPL v3",