Fix typos with codespell tool

This includes fixing convert_method_to_flag so it recognizes gauss parameter properly instead of silently defaulting to bicubic. Previously the only way to use Gaussian resizing method was to call the function incorrectly, with misspelled guass parameter.

The proccess was manually supervised, so there's no need to worry that meaningful code has been corrupted. (Disclaimer: there still might be some other typos or linguistic mistakes not detected by codespell, so watch out.)