Write a function named cumulative that accepts as a parameter an array of numbers, and
modifies it so that each element contains the cumulative sum of the elements up through that index.
For example, if the array passed contains [1, 1, 2.5, 3, 5], your function should modify
it to store [1, 2, 4.5, 7.5, 12.5].
You may assume the array passed is non-null and contains only number values.
A note about references in PHP: In order to write a function passes a parameter as
reference (thus modifying its state), you'll need to prepend "&" to the variable declaration
in the function header. For example, a function foo that modifies the state
of an array parameter may be defined as:
function foo(&$arr) { ... }
