Add a DomainException that avoids using null in functions.

core/src/main/java/org/nwapw/abacus/number/DomainException.java

@@ -0,0 +1,24 @@
+package org.nwapw.abacus.number;
+
+/**
+ * Exception thrown if the function parameters do not match
+ * requirements.
+ */
+public class DomainException extends RuntimeException {
+
+    /**
+     * Creates a new DomainException.
+     * @param reason the reason for which the exception is thrown.
+     */
+    public DomainException(String reason) {
+        super(reason);
+    }
+
+    /**
+     * Creates a new DomainException with a default message.
+     */
+    public DomainException(){
+        this("Domain Error");
+    }
+
+}