[feature] Add a shell.nix pinned on argopkgs

Full nix definition of a shell environment capable of compiling,
debugging, style-check, and building docs for AML.

Package definition pinned from the argopkgs master.

shell.nix

+# development shell, includes aml dependencies and dev-related helpers
+# defined by argopkgs nix pkg record
+{ pkgs ? import (builtins.fetchTarball "https://xgitlab.cels.anl.gov/argo/argopkgs/-/archive/master/argopkgs-master.tar.gz") {} }:
+with pkgs;
+pkgs.mkShell {
+	name = "aml";
+	nativeBuildInputs = [ autoreconfHook pkgconfig ];
+        buildInputs = [
+          # dependencies for the code
+          hwloc
+          numactl
+          # deps for docs
+          graphviz
+          doxygen
+          python3Packages.sphinx
+          python3Packages.breathe
+          python3Packages.sphinx_rtd_theme
+          # deps for debug
+          gdb
+          valgrind
+          # style checks
+          (clang-tools.override (o:{llvmPackages=pkgs.llvmPackages_7;}))
+          llvmPackages_7.clang-unwrapped.python
+        ];
+        CFLAGS = "-std=c99 -pedantic -Wall -Wextra -Werror -Wno-unused-but-set-parameter -Wno-builtin-declaration-mismatch";
+}