Add "Alexandria" provider (#88)

* Add Alexandria provider

* Replace dcgat with alexandria

* Move provider data to versioned dir

src/index-metadbs/alexandria/v1/info.json

@@ -0,0 +1,42 @@
+{
+   "meta" : {
+       "api_version" : "1.0.0",
+       "query" : {
+           "representation" : "/info"
+       },
+       "more_data_available" : false,
+       "schema" : "https://schemas.optimade.org/openapi/v1.0/optimade_index.json"
+   },
+   "data" : {
+      "attributes" : {
+         "available_api_versions" : [
+            {
+               "version" : "1.0.0",
+               "url" : "https://providers.optimade.org/index-metadbs/alexandria/v1/"
+            }
+         ],
+         "is_index" : true,
+         "formats" : [
+            "json"
+         ],
+         "entry_types_by_format" : {
+            "json" : []
+         },
+         "available_endpoints" : [
+            "info",
+            "links"
+         ],
+         "api_version" : "1.0.0"
+      },
+      "type" : "info",
+      "relationships" : {
+         "default" : {
+            "data" : {
+               "id" : "alexandria",
+               "type" : "links"
+            }
+         }
+      },
+      "id" : "/"
+   }
+}

src/index-metadbs/alexandria/v1/links.json

@@ -0,0 +1,34 @@
+{
+   "meta" : {
+       "api_version" : "1.0.0",
+       "query" : {
+           "representation" : "/links"
+       },
+       "more_data_available" : false,
+       "schema" : "https://schemas.optimade.org/openapi/v1.0/optimade_index.json"
+   },
+   "data" : [
+      {
+         "id" : "alexandria",
+         "type" : "links",
+         "attributes" : {
+            "name": "alexandria",
+            "description": "A new dataset of 415k stable and metastable materials calculated with the PBEsol and SCAN functionals",
+            "base_url": "https://alexandria.odbx.science",
+            "homepage": "https://doi.org/10.24435/materialscloud:5j-9m",
+            "link_type": "child"
+         }
+      },
+      {
+         "id" : "optimade-providers",
+         "type" : "links",
+         "attributes" : {
+            "name": "OPTIMADE Providers Index Meta-Database",
+            "description": "The list of OPTIMADE providers maintained by Materials-Consortia",
+            "base_url": "https://providers.optimade.org",
+            "homepage": "https://www.optimade.org",
+            "link_type": "providers"
+         }
+      }
+   ]
+}

src/links/v1/providers.json

@@ -30,6 +30,17 @@
         "link_type": "external"
       }
     },
+    {
+      "type": "links",
+      "id": "alexandria",
+      "attributes": {
+        "name": "Alexandria",
+        "description": "A collection of databases from the group of Prof Miguel A. L. Marques at Ruhr University Bochum.",
+        "base_url": "https://providers.optimade.org/index-metadbs/alexandria",
+        "homepage": "http://tddft.org/bmg",
+        "link_type": "external"
+      }
+    },
     {
       "type": "links",
       "id": "cod",