move trace merging/trimming to plotting

avoids these operations for every incoming packet instead, only when
plotting is done instead
allow float backtrace time (for shorter multichannel live monitor)
add case switch for multichannel plot
refactoring, channel selection via command line arguments changed to
seedlink format
refactor README, docstring
tweak multichannel plot
Merge remote-tracking branch 'upstream/master' into multichannel
update matplotlib dependency in README
Merge remote-tracking branch 'upstream/master' into multichannel

Conflicts:
	seedlink_plotter/seedlink_plotter.py
shade every second axes light gray
Merge branch 'master' into multichannel

Conflicts:
	seedlink_plotter/seedlink_plotter.py
add empty plots for channels that don't have data
Merge pull request #6 from megies/multichannel

Multiple stations in seedlink plotter