Bump NEST Simulator version to 3.7_rc1

Release candidate for upcoming NEST 3.7

packages/nest/package.py

@@ -28,6 +28,7 @@ class Nest(CMakePackage):
     maintainers = ['terhorst']
 
     version('master', branch='master')
+    version('3.7_rc1',    sha256='6f5948ac717d4b66c84ed53ba5c9b0e44e4e9cc3c1f3d15d8d8f21024313061e')
     version('3.6',    sha256='68d6b11791e1284dc94fef35d84c08dd7a11322c0f1e1fc9b39c5e6882284922')
     patch('nest-simulator-3.6-p1-CxxRealPath.patch', when='@3.6')
     version('3.5',    sha256='3cdf5720854a4d8a7d359f9de9d2fb3619a0be2e36932028d6940360741547bd')