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/finat.nix

@@ -0,0 +1,25 @@
+{ lib, callPackage, fetchFromGitHub, pythonPackages
+, fiat }:
+
+pythonPackages.buildPythonPackage rec {
+  version = "fecac5b4d4ecf4b02b1ee440bdc4a57bfa9f1d87";
+  name = "FInAT-${version}";
+
+  src = fetchFromGitHub {
+    owner = "FInAT";
+    repo = "FInAT";
+    rev = "${version}";
+    sha256 = "1ad7vzpvlwmx5pgmvj2v8s6n5kdrz02j7wzj25wn974a2ff6pvvd";
+  };
+
+  propagatedBuildInputs = [
+    pythonPackages.numpy
+    fiat
+  ];
+
+  meta = with lib; {
+    homepage = "https://github.com/FInAT/FInAT";
+    description = "FInAT is an attempt to provide a more abstract, smarter library of finite elements.";
+    license = licenses.mit;
+  };
+}