Add osmium download-changes tool

[nextstopsun]

The last bit missing in osmium tool is ability to download and cumulate planet changesets like osmupdatedoes.
I guess if this can be done in some form with pyosmium, it would be great to port it from there.
If this tool is in place, then my whole workflow involving ogr2poly, osmupdate, osmfilter, osmconvert and would be replaced with just osmium, making it a universal tool, similar to osmosis, but times faster.

[joto]

I totally agree, I would like this, too. But it is unlikely to happen in the short run. The main reason is that there isn't a decent and widely available HTTP library for C++ that would make this a) easy to write for me and b) easy to compile for everybody. I recommend the programs supplied with Pyosmium which can do this and are really easy to use.

[nextstopsun]

Doesn't osmium depend on curl?

[joto]

Osmium can optionally use the curl binary for reading OSM files from the web. But this has always been somewhat of a hack. For one, detecting error conditions when using an external program is problematic. So I don't want to use that kind of solution for downloading change files, possibly loosing data in the process. And it doesn't work on Windows.

I have thought about using the libcurl library internally, but the interface for streaming use is horribly complicated and I couldn't figure out how to use it.

[joto]

This functionality is available in PyOsmiums pyosmium-get-changes.py and pyosmium-up-to-date.py which I recommend to use.