Expose the <bounds> element in osmxml

[WillSewell]

OSM XML files have a <bounds> as one of the first elements. This is not currently exposed by the osmxml.Scanner. I think there are two ways of doing this:

1. Object returns an interface{} instead of osm.Object, so it can return *osm.Bounds
2. Add a new Bounds method, a bit like osmpbf.Scanner.Header. This might be nicer from an API point of view, but introduces significantly more complexity. E.g. the osmxml.Scanner now needs two states: the state when it is scanning for the <bounds> and the state where it is doing regular scanning with Scan. This is tricky to get right because it's not well-defined where in the document the <bounds> element should be. We could define it as "before an element recognised by the Scan method. But this would require some lookahead (in case the element doesn't exist. (an option is to require all documents to contain a <bounds> element at a particular location).

@paulmach have you thought about this at all?

For reference, an example OSM XML file with <bounds>:

<?xml version="1.0" encoding="UTF-8"?>
<osm version="0.6" generator="Overpass API 0.7.55.7 8b86ff77">
<note>The data included in this document is from www.openstreetmap.org. The data is made available under ODbL.</note>
<meta osm_base="2019-09-12T17:16:02Z"/>

  <bounds minlat="51.4072000" minlon="-0.2907000" maxlat="51.5913000" maxlon="0.1659000"/>

  <node id="1" lat="51.4779481" lon="-0.0014863" version="20" timestamp="2018-10-31T10:20:19Z" changeset="64040630" uid="1778799" user="SomeoneElse_Revert"/>
  <node id="78112" lat="51.5269760" lon="-0.1457924" version="3" timestamp="2018-10-15T14:49:44Z" changeset="63545352" uid="486343" user="peregrination"/>
...

[paulmach]

Another option is to just make osm.Bounds and osm.Object by adding a TypeObject and ObjectID functino that return "bounds/0" or something.

I haven't thought about this for a while so I may be missing something but that goes better with what I was trying to do with osm.Object. At first I just had Feature and Element then I needed to parse users and realized my abstraction wasn't complete so I created object to contain them. Seems like added Bounds to Object makes sense.

What do you think about that solution?

[WillSewell]

That would certainly be a lot simpler to implement, although it feels a bit hacky having osm.Bounds implement osm.Object even though it doesn't have an id. For that reason I think just returning an interface{} is better, but I don't feel too strongly one way of the other.

What do you prefer? I'm happy to open a PR.