fixed Map imports in tutorial

The general tutorial still had the old-style (Map) imports in it. I fixed the imports, together with the explanations that are linked to the respective code examples. Also the sentence

> `Map()` takes either a filename, list of filenames data array and header.

sounds weird to me, but I wasn't sure whether it was wrong grammar or not. Shouldn't it be of the form "… either an x, y or z."?