Changed SRTM download urls

[matthiasdusch]

• Closes SRTM download-links are dead #504
• Tests already exist

Changes the dead 'droppr.org' url to 'http://srtm.csi.cgiar.org/SRT-ZIP/SRTM_V41/SRTM_Data_GeoTiff/' as proposed by @DavidParkes.

A test to download a SRTM file already exists (test_srtmdownload).
But this test will not download the file if it still exits in the cache. This should probably be adapted.

[fmaussion]

A test to download a SRTM file already exists (test_srtmdownload).
But this test will not download the file if it still exits in the cache. This should probably be adapted

It should, if you either comment out the @is_download decorator or set

oggm/oggm/tests/__init__.py

Line 42 in bb666ca

RUN_DOWNLOAD_TESTS = False

to True.

This is for testing locally though, we don't download files from Travis

[matthiasdusch]

My bad. I did not realise the tearDown part of the testcase, which actually deletes the download folder at the end of the test. So cache should not be a problem.

[fmaussion]

Thanks! Going to merge this but during the time Timo is on holidays the cluster cache won't be updated, which is bad -> @OGGM/oggm-devs don't update OGGM on the cluster, or revert this PR temporarily.