Add Nix/Flake support

- Use tweag/opam-nix for supporting and building Owl
- Add 'owl' as a default library to build

.gitignore

@@ -25,4 +25,12 @@ setup.log
 *.install
 
 # editor config files
-.vscode
+.vscode
+
+# Ignore direnv files
+.envrc
+.direnv/
+
+# Ignore Nix/Flake related files/folders
+result
+result-*

flake.nix

@@ -0,0 +1,61 @@
+{
+  inputs = {
+    opam-nix.url = "github:tweag/opam-nix";
+    flake-utils.url = "github:numtide/flake-utils";
+    nixpkgs.follows = "opam-nix/nixpkgs";
+  };
+  outputs = { self, flake-utils, opam-nix, nixpkgs }@inputs:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+        on = opam-nix.lib.${system};
+        localPackagesQuery = builtins.mapAttrs (_: pkgs.lib.last)
+          (on.listRepo (on.makeOpamRepo ./.));
+        devPackagesQuery = {
+          # You can add "development" packages here. They will get added to the devShell automatically.
+          ocaml-lsp-server = "*";
+          ocamlformat = "*";
+        };
+        query = devPackagesQuery // {
+          ## You can force versions of certain packages here, e.g:
+          ## - force the ocaml compiler to be taken from opam-repository:
+          ocaml-base-compiler = "*";
+          ## - or force the compiler to be taken from nixpkgs and be a certain version:
+          # ocaml-system = "4.14.0";
+          ## - or force ocamlfind to be a certain version:
+          # ocamlfind = "1.9.2";
+          ocaml-compiler-libs = "*";
+          alcotest = "*";
+          dune-configurator = "*";
+          bigarray-compat = "*";
+        };
+        scope = on.buildOpamProject' { } ./. query;
+        overlay = final: prev:
+          {
+            # You can add overrides here
+            # dune = prev.dune.overrideAttrs (_: { buildPhase = "make && make install"; });
+          };
+        scope' = scope.overrideScope' overlay;
+        # Packages from devPackagesQuery
+        devPackages = builtins.attrValues
+          (pkgs.lib.getAttrs (builtins.attrNames devPackagesQuery) scope');
+        # Packages in this workspace
+        packages =
+          pkgs.lib.getAttrs (builtins.attrNames localPackagesQuery) scope';
+      in {
+        legacyPackages = scope';
+
+        #inherit packages;
+
+        ## If you want to have a "default" package which will be built with just `nix build`, do this instead of `inherit packages;`:
+        packages = packages // { default = packages.owl; };
+
+        devShells.default = pkgs.mkShell {
+          inputsFrom = builtins.attrValues packages;
+          buildInputs = devPackages ++ [
+            # You can add packages from nixpkgs here
+            pkgs.openblas
+          ];
+        };
+      });
+}