Support parsing of <center> element for ways (XML only)

[gregrs-uk]

The 'out center' Overpass QL option provides a centroid for each way in a

element. I've added support for parsing the element, but N.B. only for ways (not relations) and only for XML (not JSON).

I've also corrected a mistake in the 'Unknown element end' KeyError message.

[coveralls]

Coverage decreased (-1.4%) to 92.893% when pulling ebaba3e on gregrs-uk:way-center into b6e2cb1 on DinoTools:master.

[phibos]

Thanks for your contribution. Some tests failed with the following error message.

UnboundLocalError: local variable 'center_lat' referenced before assignment

Can you please provide a query to test your changes and/or write a test.

[coveralls]

Coverage decreased (-1.2%) to 93.086% when pulling 0df600d on gregrs-uk:way-center into 9698898 on DinoTools:master.

[coveralls]

Coverage decreased (-1.2%) to 93.086% when pulling 0df600d on gregrs-uk:way-center into 9698898 on DinoTools:master.

[coveralls]

Coverage decreased (-0.03%) to 94.266% when pulling b40afd5 on gregrs-uk:way-center into 9698898 on DinoTools:master.

[coveralls]

Coverage decreased (-0.03%) to 94.266% when pulling 82c2339 on gregrs-uk:way-center into 9698898 on DinoTools:master.

[gregrs-uk]

I've fixed __init__.py in a few places.

I've also added a test involving another XML file. This represents the same way used in another test, but is what would be returned if the out center Overpass QL option is used.

N.B. the query I used also included the (._;>;); option, which means that the XML includes <node> elements. This is probably good because it means we can test the overpy.Way.nodes property. However, it's not compulsory to return <node> elements in the XML if the out center option is used on its own.

[phibos]

Thanks for your contribution. I had a quick look at your code. But I will do some more tests before merging it.

[Gloix]

Hi, @phibos could you test this PR?

[phibos]

I have merged the source with the latest master branch and have created a new pull request #47