Rename ol.xml.pushAndParse to ol.xml.pushParseAndPop

Following discussions with @fredj, we agreed that `pushParseAndPop` is a better name given [what this function does](https://github.com/twpayne/ol3/blob/432d10a45718b8f798fbf2bdce6f7bb9f05de450/src/ol/xml.js#L203).