ImmutableDoubleValueImpl needs as* methods

When trying to extract floating point numbers from values, I get an MessageTypeCastException.

This because the ImmutableDoubleValueImpl as no as\* methods, which makes it hard to get the values out.

```
switch (value.getValueType())
{
    case FLOAT:
        return value.asNumberValue().toFloat();
    default: throw new Exception();
}
```
