todos for new calibration module in obspy.signal

[megies]

Some things need to be done to finalize the integration of the new obspy.signal.calibration module:

• add module to signal API front page module list DONE
• maybe add a subsection describing it to the signal API front page
• add an example to the Tutorial page
• adjust the tests, currently signal tests seem to get skipped if the import of _spectral_helper() fails (needs relatively new matplotlib) DONE

[megies]

In [2583/obspy]:

signal.calibration: adding it to signal API module list (see #270)

[megies]

In [2584/obspy]:

signal.calibration/core.util: (see #270)

• trying to regain consistent import of matplotlib / correct obspy.signal testing
• moving MATPLOTLIB_VERSION number generation / variable to core.util
• according to blame history of  https://github.com/matplotlib/matplotlib/blame/master/lib/matplotlib/mlab.py and  http://matplotlib.sourceforge.net/_static/CHANGELOG it seems that matplotlib.mlab._spectral_helper is available from matplotlib 0.98.4 onward
• setting this version as minimum for usage of calibration routines
• note that the source code of mlab says: "It is *NOT* meant to be used outside of mlab"
• tests run again with old matplotlib versions
• changing tests so that this particular test is skipped if the matplotlib version is not sufficiently new
• not sure if this should be changed to let the test fail with an ImportError?

[krischer]

In [2587/obspy]:

• save_data kwarg now actually defaults to True ...
• For a discussion of this and the previous commit see #270.