Revert "DataTab.process{_state}: improved multiprocessing"

This reverts commit 1c6c9705d4a019bebd84eb1a49f9d70d18ad9543.

    - Pool causes serious problems with accessing globalv data in sub
      processes, needs a more through tests, or a different implementation

Conflicts:
	gwsumm/tabs/data.py
SpectrumDataPlot: updated Spectrum.read for new GWpy
DataTab.process: fixed mp queue problems