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/pyop2.nix

@@ -0,0 +1,45 @@
+{ lib, callPackage, fetchFromGitHub, pythonPackages
+, openmpi
+, coffee, petsc, petsc4py, mpi4py }:
+
+pythonPackages.buildPythonPackage rec {
+  version = "98aab7bbbab2945317c1b9ecf5885652cf20e709";
+  name = "PyOP2-${version}";
+
+  src = fetchFromGitHub {
+    owner = "OP2";
+    repo = "PyOP2";
+    rev = "${version}";
+    sha256 = "1hw8wpbfgw8j48jfhv72pxq33as2mkw9a9k2qg0607d77agjmpz5";
+  };
+
+  buildInputs = [
+    openmpi
+    pythonPackages.cython
+    pythonPackages.pytest
+    pythonPackages.flake8
+    pythonPackages.pycparser
+    petsc
+  ];
+
+  propagatedBuildInputs = [
+    pythonPackages.six
+    pythonPackages.numpy
+    pythonPackages.decorator
+    mpi4py
+    coffee
+    petsc4py
+  ];
+
+  postPatch = ''
+    sed -i -e 's|self\._cc = os\.environ\.get(ccenv, cc)|self._cc = '\'"$(type -p mpicxx)"\''' if cpp else '\'"$(type -p mpicc)"\'''|' pyop2/compilation.py
+    sed -i -e 's|self\._ld = os\.environ\.get('\'''LDSHARED'\''', ld)|self._ld = None|' pyop2/compilation.py
+    sed -i -e 's|os\.environ\['\'''PETSC_DIR'\'''\]|'\'''${petsc}'\'''|' pyop2/utils.py
+  '';
+
+  meta = with lib; {
+    homepage = "http://op2.github.io/PyOP2/";
+    description = "Framework for performance-portable parallel computations on unstructured meshes.";
+    license = licenses.bsd3;
+  };
+}