Ensure state is cleared during setMetadata

APNG, ICS and Micromanager Formats all have readers that maintain state
(e.g. last plane read).

When a reader is used on multiple datasets, this state is not always
cleared, leading to situations where the Reader may try to open the
dataset incorrectly.

This issue should be resolved for these formats now, as they all clear
their state during setMetadata calls.

Closes #57