Merge branch 'add-extras-requires' into 'master'

Add an extras requires to enable downstream packages to depend on bilby[gw]

See merge request lscsoft/bilby!939

setup.cfg

@@ -13,4 +13,11 @@ addopts =
     --ignore test/integration/sampler_run_test.py
 
 [metadata]
+name = bilby
 license_file = LICENSE.md
+
+[options.extras_require]
+gw =
+	astropy
+	gwpy
+	lalsuite