IterableMax.compute( Iterable ) always returns DoubleType regardless of input generic parametr

This is a follow-up to a [bug report on the ImageJ forum](http://forum.imagej.net/t/stats-max-op-returns-wrong-result-type/11447) and mostly c&p of [my post in that thread](http://forum.imagej.net/t/stats-max-op-returns-wrong-result-type/11447/3?u=hanslovsky):

MWE
```java
import net.imagej.ImageJ;
import net.imglib2.img.array.ArrayImg;
import net.imglib2.img.array.ArrayImgs;
import net.imglib2.img.basictypeaccess.array.FloatArray;
import net.imglib2.type.numeric.real.DoubleType;
import net.imglib2.type.numeric.real.FloatType;

public class Dummy
{

public static void main( final String[] args )
{
final ImageJ ij = new ImageJ();
final ArrayImg< FloatType, FloatArray > floats = ArrayImgs.floats( 1, 2, 3 );
final DoubleType doubleResult = ( DoubleType ) ( Object ) ij.op().stats().max( floats );
final FloatType floatResult = ij.op().stats().max( floats );
System.out.println( doubleResult + " " + floatResult );
ij.context().dispose();
}

```
throws this exception:
```
Exception in thread "main" java.lang.ClassCastException: net.imglib2.type.numeric.real.DoubleType cannot be cast to net.imglib2.type.numeric.real.FloatType
at Dummy.main(Dummy.java:16)
```
As you can see, that weird `( DoubleType ) ( Object )` cast works in line 15, but the expected correct line 16
```java
final FloatType floatResult = ij.op().stats().max( floats );
```
throws an exception.

I am not an imagej-ops expert by any means but here are my 2 cents: The culprit is probably [`AbstractStatsOp.createOutput`](https://github.com/imagej/imagej-ops/blob/caecb88fafcb5b7d769fdaffca724a09ed943896/src/main/java/net/imagej/ops/stats/AbstractStatsOp.java#L52): It always creates a `DoubleType`. [`IterableMax`](https://github.com/imagej/imagej-ops/blob/caecb88fafcb5b7d769fdaffca724a09ed943896/src/main/java/net/imagej/ops/stats/IterableMax.java) could override `createOutput`, e.g. something like this:
```java
@Override
public T createOutput( final Iterable< T > input )
{
return input.iterator().next().createVariable();
}
```
The problem here is that this will throw an exception if `input` has zero elements.

Run the supplied Java MWE first, then inspect AbstractStatsOp.createOutput and IterableMax, the two locations named in the report. Check how max handles FloatType input and empty Iterable input; the issue is complete when the result type matches the input generic without introducing an empty-input exception.

Written by the indexing model from the issue text.