Fixed problem_tab naming conventions

Fixed all naming convention warnings in problem_tab. 

added x,y,z to good names in pylintrc because I couldn't think of a better name for x y z.

Convetion is usually [a-z_][a-z0-9_]{2,30} x,y,z violate this because they are less than 2 chars. i,j,k,_ are on good name list already.