Make LinuxFS the default GPIO provider for Raspberry Pi + extend with helper functions

[FDelporte]

Extending the Raspberry Pi plugin will enable us to achieve following goals

• Make it easier for the user to configure GPIOs
• Link PWM channels with the correct GPIO depending on the type of Raspberry Pi
• Alert the user of wrong settings, e.g. "PWM channel on GPIO X is not possible"

Part of this can be achieved by merging the project https://github.com/FDelporte/PiHeaders into the Raspberry Pi plugin. With some additional helper functions we would be able to return the right board type with all the related information regarding number of pins, type of pins, etc.

Some of these are related to the ongoing work in #212.