Fixed join when col and row are specified

[dinisov]

Hi. This is my first pull request :)

There was a problem when performing a join() on a set of tiles generated when the number of cols and rows is specified, as opposed to calculated automatically from the total number of tiles. So for instance if col=6 and row=4 then join() would call calc_columns_rows(len(tiles)) at some point which would return col=4 and row=6, since the automatic calculations default to a higher number of rows.

I changed calc_columns_rows() to calculate rows and columns based on tile.coords to make it fool proof. As for the automatic calculations I moved them to the slice() function since in all likelihood they will only be run once. My code is not very elegant but I could not think of an obvious way to add cols and rows as global variables and did not want to add them to every Tile.