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_48/sol1.py

@@ -1,21 +1,29 @@
-from __future__ import print_function
-'''
+"""
 Self Powers
 Problem 48
 
 The series, 11 + 22 + 33 + ... + 1010 = 10405071317.
 
 Find the last ten digits of the series, 11 + 22 + 33 + ... + 10001000.
-'''
+"""
 
 try:
-	xrange
+    xrange
 except NameError:
-	xrange = range
-
-total = 0
-for i in xrange(1, 1001):
-	total += i**i
+    xrange = range
 
 
-print(str(total)[-10:])
+def solution():
+    """Returns the last 10 digits of the series, 11 + 22 + 33 + ... + 10001000.
+
+    >>> solution()
+    '9110846700'
+    """
+    total = 0
+    for i in xrange(1, 1001):
+        total += i ** i
+    return str(total)[-10:]
+
+
+if __name__ == "__main__":
+    print(solution())