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 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).
Replaced some docstrings.

Smaller and older (more stable) requests in the the doctests.
I also made some changes to the documentation structure.
Much faster tests.

I carefully removed some parts and sped up some others as the tests took
too long. I don't think we lost any test coverage but this makes testing
the FDSN client actually practical.
Removing debug statements.
The attach response option will now request once per channel.

This will be much faster on average compared to the previous
solution. The previous solution requested once per station
for all time frames resulting in potentially hundreds of
downloaded responses for one trace making in unbearably
slow in some cases. This should likely be backported to the
releases branch.
Merge branch 'master' into fdsn_tests_speedup
Small bugfix in station plotting.
Added station plot to the FDSN documentation.
Added more station plots to the FDSN documentation.