A teacher has three exam grades for all the members of her class, and she
worries that the grades are too low. You need to
complete the method, named testAverage,
in the class named Grades.java.
There are two parameters to this method: the first is an integer representing
the number of students in the class and, the second is a two-dimensional
array of integers. In other words, each row represents the three grades
for one student, while the columns represent the three sets of test grades.
The return value should be the number of students with an average
exam grade below seventy. The grades should be treated as
`double` variables.

For example, consider the grades for the four students in the following 2-D array,

[ [88, 84, 89] [76, 64, 67] [85, 76, 79] [95, 90, 91] ]

The first student has an exam average of 87.0 (= (88+84+89)/3 = 261/3), the second student has an exam average of 69.0 (= (77+64+67)/3 = 207/3), the third student has an exam average of 80.0 (= (85+76+79)/3 = 240/3), while the fourth student has an exam average of 92.0 (= (95+90+91)/3 = 276/3). This means that the number of students, in this group, that have an exam average less than 70.0 is only one.