Add support multiple outputs in FunctionEvaluator

[jobergum]

A model might have several outputs.

Current API only supports one output per FunctionEvaluator.

FunctionEvaluator evaluator  = modelsEvaluator.evaluatorOf("reader", "output_0");
FunctionEvaluator evaluator1  = modelsEvaluator.evaluatorOf("reader", "output_1");
FunctionEvaluator evaluator2  = modelsEvaluator.evaluatorOf("reader", "output_2");

Not supporting several outputs in one evaluator is not optimal for models with multiple outputs as you need to invoke the model several times, once for each result. Something like

FunctionEvaluator evaluator  = modelsEvaluator.evaluatorOf("reader", "output_0","output_1", "output_2");

[jobergum]

Is this now complete with Vespa 8 @lesters ? If so, we should implement the reader using stateless evaluation for the dpr sample app.

[lesters]

@jobergum Some work still remains. I'll get back to this ASAP.

[baldersheim]

soon timed out