#4 Extracting only slice 0 fails

[Terese Nilsson]

Using version 1.1.0, the function -slice can not extract only slice number 0 anymore with the command "-slice z 0".
This is because pos_step is given the same value as pos_first and pos_last, i.e. 0 and makes the for loop on line 208 run forever. This can easily be fixed by just removing pos_step from line 165.