Made changes to Options, RollNo. check, CGPA Check

[Sumanth1997]

I made the following changes,

1. Now, when the code is run, the main function will be executed, which will first give the user options to select from.
2. Added a check to see if the input for the roll number is an integer; if not, an error message will be printed.
3. Added a check to see if the input for CGPA is a decimal number; if not, an error message will be printed.
4. In the option "Remove student by RollNo", if the student is not found, an error message will be shown stating the same.
5. In the option "View student by Name", if the student is not found, an error message will be shown stating the same.
6. Added a new option, "Option 5", to view the list of all students.