Add a FLOAT_FORMAT creation option to ASCII Grid writer

[cdestigter]

By default the AAIGrid float format string is "%6.20g"

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

That's a start, but we're finding it falls short. We want to be able to remove the extra spaces per pixel, while at the same time stripping unneccessary decimal components for ints.

For maximum flexibility we'd like to do this:

-co FLOAT_FORMAT=%.6g

I'll attach a patch shortly that addresses this.