attemptConversionToDouble

NSString-attemptConversionToFloat returns CGFloat which is confusing because it can be double on 64bit systems.
But actually floatValue is taken from string. So this method always return float precision value even if it is 'wrapped' into a double. So we loose precision on 64bit systems.

I have made small amendments so there are 2 methods now:
-(float)attemptConversionToFloat
-(double)attemptConversionToDouble

This change it is backward compatible because even old call -(CGFloat)attemptConversionToFloat actually returned float or double but always with float precision.