Created Budget interface

src/main/java/no/ntnu/idatt1002/demo/data/Budget/Budget.java

+package no.ntnu.idatt1002.demo.data.Budget;
+
+public interface Budget {
+
+    void addToBudget(int AmountToAddToBudget);
+    void setAmountBudget(int amountBudget);
+    void setBudgetPeriod(int budgetPeriod);
+    void subtractFromBudget(int amountToSubtractFromBudget);
+//    public void divideBudgetInDifferentCategories(ExpenseCategory category);
+
+}