Add a +devmode variant to yt

When specifying +devmode when installing yt, a link to the source
directory will be used instead of creating an egg. This eases the
development process as changes you make in the yt source directory will
be instantly available when building the package as a diy.

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

@@ -53,6 +53,7 @@ class PyYt(Package):
     variant("astropy", default=True, description="enable astropy support")
     variant("h5py", default=True, description="enable h5py support")
     variant("scipy", default=True, description="enable scipy support")
+    variant("devmode", default=False, description="enable development mode")
 
     extends("python")
 
@@ -68,7 +69,10 @@ class PyYt(Package):
     depends_on("python @2.7:2.999,3.4:")
 
     def install(self, spec, prefix):
-        setup_py("install", "--prefix=%s" % prefix)
+        if '+devmode' in spec:
+            setup_py("develop", "--prefix=%s" % prefix)
+        else:
+            setup_py("install", "--prefix=%s" % prefix)
         self.check_install(spec, prefix)
 
     def check_install(self, spec, prefix):