Added new function to get a raw value to send to openDAQ from a value in volts.

Modified "set_analog" and "load_signal" to use the new function instead of making calculations.

Adapted to new firmware version.
Added methods arguments validation
Merge pull request #2 from armandovincelle/develop

Added new function to get a raw value to send to openDAQ from a value in...
Added more documentation: usage.rst and quickstart.rst