integrate a mininum weight for the average dossier weight

before this commit, the average dossier weight took account only pieces
justificatives. With this commit, we add a minimum weight for other
files included in an archive like pdf_export, log operations,
attachments added to traitements. This minimum weight is set arbitrary,
from the observation of some random procedures in production

spec/models/procedure_spec.rb

@@ -1026,7 +1026,7 @@ describe Procedure do
     end
 
     it 'estimates average dossier weight' do
-      expect(procedure.reload.average_dossier_weight).to eq 5
+      expect(procedure.reload.average_dossier_weight).to eq(5 + Procedure::MIN_WEIGHT)
     end
   end