NormalisableRange needs an option to store a normalized logharitmic value (e.g. for Frequencies)

Here is some sample conversion code from my internal library to store frequencies in a normalized range between 0.f and 1.f

```
class LogFrequency_normalized
{
public:
LogFrequency_normalized(double minFrequency_, double maxFrequency_)
{
setMinMaxHz(minFrequency_, maxFrequency_);
};
void setMinMaxHz(double minFrequency_, double maxFrequency_)
{
minFrequency=minFrequency_;
maxFrequency=maxFrequency_;
minFrequencyLog=log(minFrequency_);
maxFrequencyLog=log(maxFrequency_);
};
double getFrequency(double normalizedLog)
{
return exp((normalizedLog*(maxFrequencyLog-minFrequencyLog))+minFrequencyLog);
};
double getNormalizedLog(double frequency)
{
if (frequency<=minFrequency)
{
return 0.;
} else
if (frequency>=maxFrequency)
{
return 1.;
} else
{
return (log(frequency)-minFrequencyLog)/(maxFrequencyLog-minFrequencyLog);
};
};
private:
double minFrequency, maxFrequency;
double minFrequencyLog, maxFrequencyLog;
};
```