rbf is now optional dependency

Author: Gabri95

README.md

@@ -172,7 +172,6 @@ rbf
 ```
 are required to use the steerable differential operators.
 
-
 Check the branch [legacy_py3.6](https://github.com/QUVA-Lab/e2cnn/tree/legacy_py3.6) for a Python 3.6 compatible version of the library.
 
 ## Installation
@@ -189,6 +188,11 @@ pip install git+https://github.com/QUVA-Lab/e2cnn
 ```
 
 
+NOTE: the ```rbf``` dependency (necessary to use differential operators) can not
+be installed automatically since PyPI does not support direct dependencies 
+(see [here](https://stackoverflow.com/a/54894359)).
+You can install ```rbf``` manually by following [these](https://github.com/treverhines/RBF) instructions.
+
 ## Cite
 
 The development of this library was part of the work done for our paper

setup.py

@@ -16,7 +16,7 @@
 setup_requires = []
 tests_require = ['scikit-learn', 'scikit-image']
 extras_require = {
-    'RBF-FD and Gaussians': ['cython', 'RBF @ git+https://github.com/treverhines/RBF']
+    # 'RBF-FD and Gaussians': ['cython', 'RBF @ git+https://github.com/treverhines/RBF']
 }
 
 with open("README.md", 'r', encoding='utf-8') as f: