Added doctest and more explanation about Dijkstra execution. (#1014)

* Added doctest and more explanation about Dijkstra execution.

* tests were not passing with python2 due to missing __init__.py file at number_theory folder

* Removed the dot at the beginning of the imported modules names because 'python3 -m doctest -v data_structures/hashing/*.py' and 'python3 -m doctest -v data_structures/stacks/*.py' were failing not finding hash_table.py and stack.py modules.

* Moved global code to main scope and added doctest for project euler problems 1 to 14.

* Added test case for negative input.

* Changed N variable to do not use end of line scape because in case there is a space after it the script will break making it much more error prone.

* Added problems description and doctests to the ones that were missing. Limited line length to 79 and executed python black over all scripts.

* Changed the way files are loaded to support pytest call.

* Added __init__.py to problems to make them modules and allow pytest execution.

* Added project_euler folder to test units execution

* Changed 'os.path.split(os.path.realpath(__file__))' to 'os.path.dirname()'

project_euler/problem_31/sol1.py

@@ -1,10 +1,5 @@
 # -*- coding: utf-8 -*-
-from __future__ import print_function
-try:
-    raw_input          # Python 2
-except NameError:
-    raw_input = input  # Python 3
-'''
+"""
 Coin sums
 Problem 31
 In England the currency is made up of pound, £, and pence, p, and there are
@@ -15,7 +10,13 @@ It is possible to make £2 in the following way:
 
 1×£1 + 1×50p + 2×20p + 1×5p + 1×2p + 3×1p
 How many different ways can £2 be made using any number of coins?
-'''
+"""
+from __future__ import print_function
+
+try:
+    raw_input  # Python 2
+except NameError:
+    raw_input = input  # Python 3
 
 
 def one_pence():
@@ -50,4 +51,21 @@ def two_pound(x):
     return 0 if x < 0 else two_pound(x - 200) + one_pound(x)
 
 
-print(two_pound(200))
+def solution(n):
+    """Returns the number of different ways can £n be made using any number of
+    coins?
+
+    >>> solution(500)
+    6295434
+    >>> solution(200)
+    73682
+    >>> solution(50)
+    451
+    >>> solution(10)
+    11
+    """
+    return two_pound(n)
+
+
+if __name__ == "__main__":
+    print(solution(int(str(input()).strip())))