Clarify the difference between row/col usage

**How could we improve the documentation?**

As @svank pointed out, the tutorial example uses row/col both to refer to the position in the small patches and the position in the full image for the functional PSFs. This is very confusing. 

I'd suggest renaming it as `i_patch` and `j_patch` versus `i_image` and `j_image` removing the `row` and `col` nomenclature altogether. 