Force at least selection two traits

Closes #115

client/pages/phenotypes.vue

@@ -33,7 +33,7 @@ Please select the traits you want to analyze jointly with the Omnibus test. A re
         </template>
         </v-data-table>
         <div align="left" justify="start">
-          <v-btn depressed color="primary" v-on:click="runAnalysis()">
+          <v-btn depressed color="primary" v-on:click="runAnalysis()" :disabled="selectedRows.length <=1">
             Launch analysis
           </v-btn>
         </div>