Dvg-pid-controller

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* Deprecated `requires.io`

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* Made the last error accessible
* Added optional argument `differential_input` to have the PID controller
regulate a specific difference with respect to the main argument
`current_input`. The specific difference that will be regulated is set by
`setpoint`.
Contributor: https://github.com/antonverburg.

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(yanked, should have been minor version increment instead of patch)

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(yanked, should have been minor version increment instead of patch)

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* DvG module filenames changed to lowercase

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* Fixed Readme typo