Allow User to Specify a Dataset as Read-Only

[cmhulbert]

Issue #191 contains a bug and an enhancement; This issue tracks the enhancement portion, so we can close the bug portion of #191

From the issue:

On top of that, give the user options to open any data set as read-only (really affects only label data). N5Writer should always be created if the user has write access, even if the user opens data as read only, in order to ensure the user can open other data sets from that same container in write mode, if desired.