Rigolwfm

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* add support for DS2000
* add command-line utility wfmconvert
* improve support for DS1000Z
* fix DS4000
* add .csv export
* add .wav export

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* fix requirement for enum by just requiring python > 3.4

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* huge change. Now using kaitai struct exclusively
* added support for 1000C, 4000, 6000 .wfm formats
* much more testing
* many api changes.

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* fix exception handling, support parsing URLs

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* improve README.md and long_description in setup.py

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* fix setup.py classifiers