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-yt/package.py

@@ -26,7 +26,7 @@
 from spack import *
 
 
-class PyYt(Package):
+class PyYt(PythonPackage):
     """Volumetric Data Analysis
 
        yt is a python package for analyzing and visualizing
@@ -54,8 +54,6 @@ class PyYt(Package):
     variant("h5py", default=True, description="enable h5py support")
     variant("scipy", default=True, description="enable scipy support")
 
-    extends("python")
-
     depends_on("py-astropy", type=('build', 'run'), when="+astropy")
     depends_on("py-cython", type=('build', 'run'))
     depends_on("py-h5py", type=('build', 'run'), when="+h5py")
@@ -67,12 +65,9 @@ class PyYt(Package):
     depends_on("py-sympy", type=('build', 'run'))
     depends_on("python @2.7:2.999,3.4:")
 
-    def install(self, spec, prefix):
-        setup_py("install", "--prefix=%s" % prefix)
-        self.check_install(spec, prefix)
-
-    def check_install(self, spec, prefix):
+    @PythonPackage.sanity_check('install')
+    def check_install(self):
         # The Python interpreter path can be too long for this
         # yt = Executable(join_path(prefix.bin, "yt"))
         # yt("--help")
-        python(join_path(prefix.bin, "yt"), "--help")
+        python(join_path(self.prefix.bin, "yt"), "--help")