Add POC for using nickel as a configuration lang

The `check.ncl` file contains the commands what we could use directly from snakemake to validate the configs and generate the json/yaml for ecg

artifact_contract.ncl

@@ -0,0 +1,59 @@
+let
+  conf = {
+    ARCHIVE_FORMATS = [ "zip", "tar" ],
+    PACKAGE_MANAGERS = [ "dpkg", "conda", "pip"] 
+  }
+in
+{
+  PackageManager = std.contract.from_predicate (
+      fun value => std.array.any (fun x => x == value) conf.PACKAGE_MANAGERS
+  ),
+  ArchiveType = std.contract.from_predicate (
+      fun value => std.array.any (fun x => x == value) conf.ARCHIVE_FORMATS
+    ), 
+  GitPackage = {
+    name
+      | doc "Name of the package for future identification"
+      | String,
+    location
+      | doc "Path where cloned in the container"
+      | String
+  },
+  MiscPackage = {
+    name
+      | doc "Name of the package for future identification"
+      | String,
+    url
+      | doc "URL of the package. Will be used to compute the hash"
+      | String,
+    type
+      | doc "Type of the archive (zip, tar, ...)"
+      | ArchiveType,
+  },
+  Artifact = {
+    artefact_url
+      | doc "URL where to download the artifact"
+      | String,
+    type
+      | doc "Type of the archive (zip, tar, ...)"
+      | ArchiveType,
+    doi
+      | doc "DOI of the artifact"
+      | String,
+    image_name
+      | doc "Name to give the image when building"
+      | String,
+    location
+      | doc "Path to the dockerfile in the artifact"
+      | String,
+    package_managers
+      | doc "Package Managers used in the container"
+      | Array PackageManager,
+    git_packages
+      | doc "Git repositories cloned in the container"
+      | Array GitPackage,
+    misc_packages
+      | doc "Misc. packages downloaded from the container"
+      | Array MiscPackage,
+  }
+}

artifacts_nickel/example.ncl

@@ -0,0 +1,14 @@
+{
+  artefact_url = "https://example.com/artifact.zip",
+  type = "zip",
+  doi = "...",
+  image_name = "image:version",
+  location = "path/to/docker/folder",
+  package_managers = [ "dpkg" ],
+  git_packages = [
+    { name = "pkg1", location = "path/to/git/repo"}
+  ],
+  misc_packages = [
+    { name = "mpkg1", url = "https://", type = "zip" }
+  ],
+}

check.ncl

@@ -0,0 +1,4 @@
+let { Artifact, .. } = import "artifact_contract.ncl" in
+(
+  (import "artifacts_nickel/example.ncl") | Artifact
+)

flake.nix

@@ -15,6 +15,7 @@
       devShells = {
         default = pkgs.mkShell {
           packages = with pkgs; [
+            nickel
             (python3.withPackages (ps: with ps; [
               requests
               pyyaml