Change x in all functions to step_number

[spsanderson]

I think we should change the variable name x in all output to be step_number

This will help with consistency and we are at an early stage so it should not have much effect. This way when random variables are created there is no name overlap or need to change to something like y1, y2, y3 instead we can just do walk_number, step_number, x, y , z

@AnttiRask what do you think? It would require a change to the visualize_walks() function but I think it brings a needed piece of consistency.

[spsanderson]

I have implemented this in #53