Add precision to CSV export to handle geocoordinates

[romainreignier]

With geographic coordinates, the precision is critical to get centimeter accuracy.

On the following image in QGis, le red dots have a precision of 6 and the blue ones have a precision of 9 digits.
For reference, the red rectangle has a width of 2 m.

According to this answer, 8 digits should be enough already but I have used 9 as a multiple of 3. But I can change it if you wish.