feat: check PMID validity

AoU-Model/src/main/java/ch/unige/aou/model/tool/ValidationTool.java

-package ch.unige.aou.service.metadata;
+package ch.unige.aou.model.tool;
+
+import java.math.BigInteger;
 
 public class ValidationTool {
 
@@ -31,4 +33,22 @@ public class ValidationTool {
   public static boolean isValidDate(String date) {
     return date.matches("^((?:19|20)[0-9][0-9])$|^(0?[1-9]|[12][0-9]|3[01])\\.(0?[1-9]|1[012])\\.((?:19|20)[0-9][0-9])$");
   }
+
+  public static boolean isValidPMID(String pmidString) {
+    BigInteger result;
+    if (pmidString.matches("\\d+")) {
+      try {
+        result = new BigInteger(pmidString);
+        if(result.equals(BigInteger.ZERO)) {
+          return false;
+        } else {
+          return true;
+        }
+      } catch (NumberFormatException e) {
+        return false;
+      }
+    } else {
+      return false;
+    }
+  }
 }