Add method for calculating children budgets.
Add method for calculating children budgets.

Add method to budgets.py calculating the budget of an account and adding
the budget of its children.

yagebu/fava,yagebu/fava,beancount/fava,aumayr/beancount-web,yagebu/fava,beancount/fava,beancount/fava,yagebu/fava,aumayr/beancount-web,beancount/fava,aumayr/beancount-web,yagebu/fava,aumayr/beancount-web,beancount/fava