PMT-16: Add Validation for Time Range to Project Entity

Signed-off-by: Dominik Säume <Dominik.Saeume@hmmh.de>

src/main/java/de/hmmh/pmt/db/Project.java

@@ -43,5 +43,10 @@ public class Project {
     private LocalDateTime plannedEnd;
 
     private LocalDateTime realEnd; // Cant be named just "end" because it's and SQL Keyword
+
+
+    public boolean isValid() {
+        return plannedEnd.isAfter(start) && (realEnd == null || realEnd.isAfter(start));
+    }
 }