Update filter.py

clarifying docs
Update filter.py
modified filter docs
Smaller and hopefully more stable FDSN examples.

Currently only for __init__.py.
Documentation changes and smaller doctests.
Documentation cleanup.
Caching of discovered services.

This means that repeated initializtion of a client with the same
base URL is cheap.
Printing debug information for the caching.
Changed service hash.
Caching the webservice versions.

This makes interactive use more convenient as the version is
accessed for various help functions.
Fixed receive buffers shared between SeedLink connections.

This should fix obspy/obspy#561.
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
Fixed decode error when parsing SeedLink packets.
Fixed print function calls.

Removed superfluous brackets and fixed prints to stderr (which
were already broken before the futurize run).
Added issue number to test case.
Merge pull request #769 from bmorg/fix_seedlink_multithread

Fixed obspy.seedlink bugs caused by mixed usage of class/instance attributes. Fixes #561.
correct some minor flake8 accidentally introduced by #769
Merge branch 'releases'

Conflicts:
	obspy/seedlink/slpacket.py