Adds self.middle_element method to pass middle_element tests:

.middle_element returns nil for an empty array
.middle_element returns the only element for an array.length == 1
.middle_element averages 2 middle elements for even-sized arrays
.middle_element returns floats when the middle element avereages a decimal

Also adds self.sum_arrays method that:

returns an empty array if both inputs are empty
returns a single value array when sum'ing two single value arrays
Adds methods to pass self.sum_arrays tests:

.sum_arrays returns an empty array if the inputs are empty
.sum_arrays returns an array with the sum of two arrays with array size 1
.sum_arrays returns an array with the sum of two arrays with larger arrays