world files and locale

[warmerdam]

When operating in the C locale, GDAL cannot read world files using commas
in place of a period as the decimal point (ie. typical of the "de" locale 
for instance). 

Please modify the worldfile reading code to support either "." or "," as the
decimal place regardless of the working locale.  Basically, use the 
CPLAtof() function in place of atof(). 

(see GDALReadWorldFile() in gdal/gcore/gdal_misc.cpp)

Make sure a test of this is added to the test suite.