Write a program that inputs the grades of all students in 3 different
classes and computes the standard deviation of these grades. The number of
students in each class may vary. The number of students in each class will be asked
from the user and subsequently that many grades will be read and recorded.
Following this the same procedure will be followed for the next class, and so on.
When all the data is ready to process, the standard deviation will be
computed and output.

Example:
Enter the number of students in class 1: 3
50
60
50
Enter the number of students in class 2: 1
80
Enter the number of students in class 3: 2
100
60
The standard deviation of grades is 19.6638.

Note: Remember that standard deviation is computed as follows:
std = sqrt(sum((X[i]-avg)^2)/(N-1))

where X[i] stands for the points in the data set, avg for the average
of all data points and N for the number of data points.

sqrt denotes the function to compute the square root of its argument
and sum denotes the sum of (X[i]-avg)^2 expressions for all i. (...)^2 means
the square of (...).

HINT and REQUIREMENT: Use an array of pointers to solve this problem.