1) Problem outline 
Write a program that computes the Caesar cipher of plain text entered by the user. The user also inputs the shift value.
A Caesar cipher is computed by assigning the values 0-25 to the letters A-Z. 
To encrypt, add the shift value to each letter value and print the letter in the shited position.
Mathematically:
    - x: the assigned value of the letter
    - n: the shift value
    - Encrypt(x) = (x + n) mod 26


2) Input format
The user enters the shift value as an integer from 0 to 25. The user then enters a string to encrypt that must be all letters. 

3) Sample input values (include both valid inputs and inputs that create exceptional conditions)
- Example 1
    - Shift: 3
    - plaintext: WE RIDE AT MIDNIGHT
- Example 2
    - Shift: 5
    - plaintext: WE RIDE AT MIDNIGHT
- Example 3
    - Shift: 5
    - plaintext: FOURTH MEAL
- Exceptional inputs:
    - Shift: A
    - Shift: 32
    - plaintext: No punctionation.


4) Exceptional conditions
- Exc1: The user enters a non-integer value for the shift
- Exc2: The user enters an integer value < 0 or > 25 for the shift
- Exc3: The user enters a plaintext string containing something other than alphabetic characters.

5) Output format
The encrypted ciphertext is printed to the screen.


6) Sample output values (include the sample output for each sample input)
- Example 1: ZH ULGH DW PLGQLJKW
- Example 2: BJ WNIJ FY RNISNLMY
- Example 3: KTZWYM RJFQ
- Exc1: "Error: Shift values can only be integers from 0-25. Try again."
- Exc2: "Error: Shift values can only be integers from 0-25. Try again."
- Exc3: "Error: Only alphabetic characters are allowed (a-z and A-Z). Try again."