change URLs to https (and more) (#1643)
I went ahead and replaced most `http://` URLs with `https://`. Eight instances of `http://` remain. - MORWiki and SLICOT do not seem to use TLS - Four instances are in the bib file which I left untouched - which leaves pyamg.org and the link to NA-digest which both didn't work when I tested them I also added a missing word while I was at it.