Abstract base class for DS

[abramovd]

• Abstract base class for Data structures. Currently used only for data_structures.stack.
• Mixin for data structures unittests to check that data structure implements required methods (might be useful for new developers)

One TODO:
I don't fully understand either time_complexities method is required or not. I can't see it in a majority of implemented data structures. I added it to base class, but only with pass in its body for now.

If such refactoring approach is ok, I can do the same for other DS classes.

[OmkarPathak]

@IanDoarn @Tjstretchalot what do you think, should we use this method for all data structures? I really like the way. Just wanted a second thought from someone.

[Tjstretchalot]

I think it belongs in the documentation, personally. It's pretty difficult to find for some algorithms, and it's less obvious we don't have it if it's in the documentation

[OmkarPathak]

@Tjstretchalot so should we incorporate the changes or not?

[Tjstretchalot]

I like the base class, I don't like the DS_Class variable and changing the base class for the tests. I'm not sure I see the point in that exactly, since you can accomplish the same thing without using DS_Class and the interface mixin is a lot harder to read

[OmkarPathak]

Okay. So sadly @abramovd we have to drop this idea. But thanks for your contribution anyway 👍