Remove code-duplication of artice price calculation

app/models/concerns/price_calculation.rb

@@ -0,0 +1,14 @@
+module PriceCalculation
+  extend ActiveSupport::Concern
+
+  # Gross price = net price + deposit + tax.
+  # @return [Number] Gross price.
+  def gross_price
+    ((price + deposit) * (tax / 100 + 1)).round(2)
+  end
+
+  # @return [Number] Price for the foodcoop-member.
+  def fc_price
+    (gross_price  * (FoodsoftConfig[:price_markup] / 100 + 1)).round(2)
+  end
+end