obspy is already requirement for seishub.core
Update seishub/plugins/seismology/README.txt
add manifest file

also minor changes to readme text file
Correctly deal with multi-channel files.

In the past all channels in each file have been returned. This was
never a problem in the usual case of single-channel files.
Small workaround for old ObsPy versions.

Necessary so that older ObsPy clients can correctly retrieve data from
multi-channel files.
Fix for previous fix...