Add PythonPackage base class

- Add a PythonPackage class with build system support.
  - Support build phases in PythonPackage
  - Add a custom sanity check for PythonPackages
  - Get rid of nolink dependencies in python packages

- Update spack create to use new PythonPackage class

- Port most of Python packages to new PythonPackage class

- Conducted a massive install and activate of Python packages.
  - Fixed bugs introduced by install and activate.

- Update API docs on PythonPackage

var/spack/repos/builtin/packages/py-astropy/package.py

@@ -25,7 +25,7 @@
 from spack import *
 
 
-class PyAstropy(Package):
+class PyAstropy(PythonPackage):
     """The Astropy Project is a community effort to develop a single core
     package for Astronomy in Python and foster interoperability between
     Python astronomy packages."""
@@ -37,7 +37,6 @@ class PyAstropy(Package):
     version('1.1.post1', 'b52919f657a37d45cc45f5cb0f58c44d')
 
     # Required dependencies
-    extends('python')
     depends_on('py-numpy', type=('build', 'run'))
 
     # Optional dependencies
@@ -55,6 +54,5 @@ class PyAstropy(Package):
     depends_on('cfitsio')
     depends_on('expat')
 
-    def install(self, spec, prefix):
-        setup_py('build', '--use-system-cfitsio', '--use-system-expat')
-        setup_py('install', '--prefix={0}'.format(prefix))
+    def build_args(self, spec, prefix):
+        return ['--use-system-cfitsio', '--use-system-expat']