Cumulative frequency and the median

You will write functions that build the cumulative frequencies of a table and find its median.

The idea. The cumulative frequency is a running total. The median class is the first class whose total reaches $\dfrac{N}{2}$, and $\text{median} = l + \dfrac{\frac{N}{2} - cf}{f} \times h$.

Your tasks.

1. In cumulative, return the list of running totals of frequencies.
2. In median_class, return the position of the median class in the list, counting from $0$.
3. In grouped_median, return the median of the table, rounded to $4$ decimals. The classes start at the values in lowers and all have the width width.