Write a program which:
1. Asks the user to enter a letter grade (A, B, C, D, F) or (a, b, c, d, f)
2. Validates that the entry is a letter grade (A, B, C, D, F) or (a, b, c, d, f)
3. The program then asks the user to enter the number of units (INTEGER) of the course, such that: 0 < UNITS < 6
4. Validates that the entry is such that: 0 < UNITS < 6
5. The program then displays the GPA for the course as a decimal number formatted with two decimal places.
6. The user must be asked if he/she wants to continue entering information or quit.
7. Comment your code
8. Use meaningful or mnemonic variable names
9. RESTRICTIONS: no global variables, no GOTO statements