Implement Three New Algorithms (#948)

* Create average_median.py

I created a program to calculate the median of a list of numbers.

* Changed Odd to Even in String

* Create decimal_to_binary.py

- Added 'conversions' folder.
- Created a decimal to binary converter.

* Implemented Decimal to Octal Algorithm

- I created a decimal to octal converter based on the converter in the TheAlgorithms/Python project.
- I added two newlines to make the output of decimal_to_binary.py better.