Is your feature request related to a problem? Please describe.
The class name of IoU metrics is "MeanIoU", which is not consistent with other metrics, like the dice metric. And it is similar to a metric handler, like the mean dice handler. This may make users confuse about the IoU handler and metric.

Describe the solution you'd like

• Change the class name of the iou metric to "IoUMetric"
• Add a "mean_iou.py" file to the handler folder, in which an IoU handler named "MeanIoU" will be implemented.

@Nic-Ma @wyli Do you think if it's ok to change like this?