Commented doctests that were causing slowness at Travis. #2 (#1041)

* 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()'

* Added Burrows-Wheeler transform algorithm.

* Added changes suggested by cclauss

* Fixes for issue 'Fix the LGTM issues #1024'.

* Added doctest for different parameter types and negative values.

* Fixed doctest issue added at last commit.

* Commented doctest that were causing slowness at Travis.

* Added comment with the reason for some doctest commented.

* pytest --ignore

* Added tests execution again.

* Had forgotten to add comment to file project_euler/problem_09/sol3.py

project_euler/problem_09/sol3.py

@@ -21,8 +21,9 @@ def solution():
      1. a**2 + b**2 = c**2
      2. a + b + c = 1000
 
-    #>>> solution()
-    #31875000
+    # The code below has been commented due to slow execution affecting Travis.
+    # >>> solution()
+    # 31875000
     """
     return [
         a * b * c