Use arduino map function

[matt-downs]

Refactor code to use the built in map function in place of the normaliseSensorData and mapValueToYPos functions.

Created as a pull request because this needs to be tested and my arduino is in my dashboard.

[matt-downs]

@half-finished Sure!

The boostPressure variable contains the pressure as an int: boostPressure = 1234 would mean a measurement of 12.34psi

To convert 1234 to 12.34 I cast the int to a float and divide by 100.

I then use the dtostrf function to convert and round the float to a char array so that it can be easily drawn to the screen. dtostrf is a built in function which you can read about here: http://www.hobbytronics.co.uk/arduino-float-vars

[half-finished]

I ended up using the formula from the data sheet, I seemed to get a more accurate reading from it than when using the map function.