Fixed spuriously declared class variables.

The way the trace variable was declared allowed for setting a
class variable, effectively rendering every getTrace() call on
every packet useless.

The blockette class variable was unused entirely.
Fixed spuriously declared class variables.

The way the selectors variable was declared led to the following bug:

>>> station1 = SLNetStation('', '', None, -1, None)
>>> station2 = SLNetStation('', '', None, -1, None)
>>> station1.appendSelectors('FOO')
1
>>> station2.getSelectors()
['FOO']
Changelog for obspy.seedlink bugfixes