implement logic in the PurchaseCalculator class

src/main/java/edu/ntnu/idi/idatt2003/gruppe42/Model/Calculator/PurchaseCalculator.java

@@ -14,21 +14,22 @@ public PurchaseCalculator(Share share) {
 
   @Override
   public BigDecimal calculateGross() {
-    return null;
+    return purchasePrice.multiply(quantity);
   }
 
   @Override
   public BigDecimal calculateCommission() {
-    return null;
+    BigDecimal commissionRate = new BigDecimal("0.01");
+    return calculateGross().multiply(commissionRate);
   }
 
   @Override
   public BigDecimal calculateTax() {
-    return null;
+    return BigDecimal.ZERO;
   }
 
   @Override
   public BigDecimal calculateTotal() {
-    return null;
+    return calculateGross().add(calculateCommission()).add(calculateTax());
   }
 }