Current implementation is buggy and hard to read.
* Negative values were raising a TypeError due to `math.sqrt`
* 1 was considered prime, it is not.
* 2 was considered not prime, it is.
The implementation has been corrected to fix the bugs and to enhance
readability.
A docstring has been added with the definition of a prime number.
A complete test suite has been written, it tests the 10 first primes, a
negative value, 0, 1 and some not prime numbers.
closes#795
* organized graph algorithms
* all graph algorithms in Graphs/ folder
* all graph algorithms are in one folder
* Rename number theory/factorial_python.py to maths/factorial_python.py
