Optimized unit changes.

- Created new updateReading() function to change the pressure value as soon as the unit is changed.
- Removed toggleUnit() function.
- Created new function in Pressure class, getPressureValue(), to return just the value of pressure without the unit.
- Replaced unit-changing tap with Spinner component for increased visibility.
- New string array file containing all the units.
- Removed some unnecessary files from the bin/ subdirectory.