DecayFunction (elasticsearch-extend 3.2 API)

```
public interface DecayFunction
```
Implement this interface to provide a decay function that is executed on a distance. For example, this could be an exponential drop of, a triangle function or something of the kind. This is used, for example, by GaussDecayFunctionBuilder.

Method Summary

All Methods Instance Methods Abstract Methods
Modifier and Type	Method	Description
`double`	`evaluate(double value, double scale)`
`Explanation`	`explainFunction(String valueString, double value, double scale)`
`double`	`processScale(double scale, double decay)`	The final scale parameter is computed from the scale parameter given by the user and a value.

- Method Detail
  - evaluate
```
double evaluate(double value,
                double scale)
```
  - explainFunction
```
Explanation explainFunction(String valueString,
                            double value,
                            double scale)
```
  - processScale
```
double processScale(double scale,
                    double decay)
```
    The final scale parameter is computed from the scale parameter given by the user and a value. This value is the value that the decay function should compute if document distance and user defined scale equal. The scale parameter for the function must be adjusted accordingly in this function
    
    Parameters:
    
    scale - the raw scale value given by the user
    
    decay - the value which decay function should take once the distance reaches this scale