Change float format string for AAIGrid writer to %.20g

[cdestigter]

The AAIGrid float format string is "%6.20g"

Firstly, the 6 doesn't really add anything, and when writing small integers it causes them to be space-padded, which is a waste of bytes.

At present AAIGrid supports "-co DECIMAL_PRECISION=10" which changes the formatstring to "%.10f".

This is a little inconsistent, and the %f causes integers to be written as "9.000000000" which is a horrible waste of bytes.

I'll attach a minimal patch shortly which fixes these.