firedrake: Moved to new overlay system and shared sub-scope

Firedrake contains a ton of inter-connected dependencies, many of
which are either firedrake specific or forked versions of upstream
packages.  Cleaner to create a firedrake sub-scope for all these.

pkgs/firedrake/petsc4py.nix

@@ -0,0 +1,31 @@
+{ lib, callPackage, fetchFromGitHub, pythonPackages
+, openmpi, hdf5, petsc }:
+
+pythonPackages.buildPythonPackage rec {
+  version = "fbe23a494ab485f44f00ee37eee9f8be8dcd9eb5";
+  name = "firedrake-petsc4py-${version}";
+
+  src = fetchFromGitHub {
+    owner = "firedrakeproject";
+    repo = "petsc4py";
+    rev = "${version}";
+    sha256 = "0172rgpyqh2ylr4wv4a79xfls8fbbdb86n4p6r7cx5kdy8d2y05b";
+  };
+
+  buildInputs = [
+    pythonPackages.cython
+    openmpi
+    hdf5
+    petsc
+  ];
+
+  propagatedBuildInputs = [
+    pythonPackages.numpy
+  ];
+
+  meta = with lib; {
+    homepage = "https://bitbucket.org/petsc/petsc4py/";
+    description = "Python bindings for PETSc.";
+    license = licenses.bsd2;
+  };
+}