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()'

data_structures/hashing/double_hash.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 
-from .hash_table import HashTable
+from hash_table import HashTable
 from number_theory.prime_numbers import next_prime, check_prime
 
 

data_structures/hashing/hash_table_with_linked_list.py

@@ -1,4 +1,4 @@
-from .hash_table import HashTable
+from hash_table import HashTable
 from collections import deque
 
 

data_structures/hashing/quadratic_probing.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 
-from .hash_table import HashTable
+from hash_table import HashTable
 
 
 class QuadraticProbing(HashTable):

data_structures/stacks/infix_to_postfix_conversion.py

@@ -2,7 +2,7 @@ from __future__ import print_function
 from __future__ import absolute_import
 import string
 
-from .Stack import Stack
+from stack import Stack
 
 __author__ = 'Omkar Pathak'