setup: add extras for inspiral-range

setup.cfg

+# https://setuptools.readthedocs.io/en/latest/userguide/declarative_config.html
+
 [aliases]
 test=pytest
 
@@ -46,6 +48,7 @@ project_urls =
 
 [options]
 packages = find:
+include_package_data = True
 python_requires = >=3.6
 install_requires =
     h5py
@@ -56,7 +59,9 @@ install_requires =
     scipy
 tests_require =
     PyPDF2
-include_package_data = True
+
+[options.extras_require]
+range = inspiral-range
 
 [options.entry_points]
 console_scripts =