'wget' dependency not mentioned in the documentation

[OliverJennrich]

Documentation should mention dependency on 'wget'. Alternatively, pul() should fail with an appropriate error message if 'wget' cannot be found/executed. At the moment it fails silently.

Suggestion:
In line 252 in catalog.py check the return of os.systeM('wget....') through os.WEXITSTATUS() and fail if it is not zero, i.e.

if os.WEXITSTATUS( os.system('wget -q --directory-prefix='+sdir+' '+surr_url)) >0:
raise ValueError("wget not installed")

[duetosymmetry]

For this one and #20, maybe we should just move away from using wget to fetch data. We can use urllib.request (the legacy function urlretrieve writes to disk for you and hasn't been deprecated yet). Do we really need to use wget?