Disallow downloading curves when Merit is disabled

It no longer makes sense to do a "real" Merit run even on scenarios with
the Merit Order disabled due to interactions with Fever (particularly
when P2H is present in the scenario). Fever will not be available if
Merit was not enable in the scenario.

Closes ETENGINE-7

app/controllers/api/v3/curves_controller.rb

@@ -8,6 +8,8 @@ class CurvesController < BaseController
         render json: { errors: ['Scenario not found'] }, status: 404
       end
 
+      before_action :merit_required
+
       # Downloads the load on each participant in the merit order as a CSV.
       #
       # GET /api/v3/scenarios/:scenario_id/merit/loads.csv
@@ -53,6 +55,12 @@ def household_heat_curves
 
       private
 
+      def merit_required
+        unless Qernel::Plugins::MeritOrder.enabled?(scenario.gql.future_graph)
+          render :merit_required, format: :html, layout: false
+        end
+      end
+
       def send_csv(name)
         send_data(
           CSV.generate { |csv| yield csv },
@@ -62,9 +70,7 @@ def send_csv(name)
       end
 
       def merit_order
-        @mo ||= Qernel::Plugins::MeritOrder.new(
-          scenario.gql.future_graph
-        ).order.calculate
+        scenario.gql.future_graph.plugin(:merit).order
       end
 
       def scenario

app/views/api/v3/curves/merit_required.html.haml

@@ -0,0 +1,72 @@
+!!!
+%html
+  %head
+    %title Merit and time-resolved calculations are not enabled
+    %meta{:content => "width=device-width,initial-scale=1", :name => "viewport"}/
+    :css
+      body {
+        background-color: #EFEFEF;
+        color: #2E2F30;
+        text-align: center;
+        font-family: -apple-system, BlinkMacSystemFont, Segoe UI, Roboto, Droid Sans, Arial, sans-serif;
+        margin: 0;
+      }
+
+      div.dialog {
+        width: 95%;
+        max-width: 43em;
+        margin: 4em auto 0;
+      }
+
+      div.dialog > div {
+        border: 1px solid #bbb;
+        border-top: #B00100 solid 4px;
+        background-color: white;
+        padding: 2em;
+      }
+
+      h1 {
+        font-size: 150%;
+        color: #9C0100;
+        line-height: 1.5em;
+      }
+
+      p {
+        line-height: 1.5;
+        padding: 0 40px;
+      }
+
+      p.back {
+        margin-top: 1.1em;
+      }
+
+      a {
+        color: #1975ca;
+        text-decoration: none;
+      }
+
+      a:hover {
+        text-decoration: underline;
+      }
+
+      .back a {
+        margin-left: -1em;
+        padding: 10px 15px;
+      }
+
+      .back a:hover {
+        background: #1975ca;
+        color: white;
+        text-decoration: none;
+      }
+  %body
+    .dialog
+      %div
+        %h1 Merit and time-resolved calculations are not enabled
+        %p
+          To download curves you must first enable the Merit Order in your
+          scenario. You can find this option in the settings menu or by
+          navigating to "Electricity balance" → "Merit order".
+        %p.back
+          %a(href="//pro.energytransitionmodel.com" onclick="window.history.back(); return false;" title="Go back")
+            ← Go back