Check attach_paz and attach_resp in obspy.sac

[megies]

Following  Stephen's mail on the users mailing list, I think we need to check both functions mentioned in the ticket title.

To me it seems that what is stored as gain by attach_paz seems to belong in sensitivity in the attached paz dictionary with gain set to 1.0. Although, I do not know much about SAC's pole/zero format and never used it much.

With attach_resp what goes into seismometer_gain should probably be stored in sensitivity. Although, the  RESP format definition states that more than one line might start with the search pattern of "B058F04" with the last being the product of everything again. I think we could use this overall product and divide it by the specified gain and store both. Again, I do not know RESP format well. This is just a very quick assessment.

Furthermore, attach_resp has no connection to SAC that I can see, so this should probably be relocated (to obspy.xseed? to obspy.core?).

[beyreuth]

In [3126/obspy]:

adding sensitivity to paz of sacio, basically closing ticket
threehunderttwenty. @Stephen: could you please double check the
results? (see #320)

[yanmail]

I think attach_resp is now obsolete since the evalresp option can be used in simulate. What's the best way to remove this function without breaking any code that might use it?