Tensor map(Tensor subspace) {
        var context = new MapEvaluationContext<NAMETYPE>();
        context.put(argName, subspace);
        return function.evaluate(context);
    }
A function suitable for use in MapSubspaces / FilterSubspaces

@author arnej
vespajlib/src/main/java/com/yahoo/tensor/functions/DenseSubspaceFunction.java