Create problemgen.py

Creates customizable addition & subtraction problems and places them by column into a CSV Excel file.

Options include:
- # of problems
- # of numbers in the problem
- # of digits in the terms
- # of subtraction terms in the problem

Current Restraints: Unable to support problems involving numbers of varying digits.

Noticed Problems: The process of placing the numbers into the CSV file can be further optimized. Currently, the program places each value one-by-one.