Improve StorageInterface, add method `get`

[Enet4]

This PR proposes a backwards-compatible extension to StorageInterface with default method get. Unlike at, get is designed to retrieve up to a single item in storage, so it does not need file system tree traversal or building up iterables and iterators.

A default implementation is provided that uses at underneath, but implementors may wish to override it for more efficiency.

This PR also includes a few quality-of-life improvements, including some clarifications to other methods in the plugin interface.

Summary

• Add default method StorageInterface#get, to retrieve a single item from storage
  • update code in core to use it where suitable
  • also correct image servlet to only query for instances to DIM providers if specified
• Fix example code in at
• Clarify in list that a trailing forward slash means that it's a directory

[Enet4]

@bastiao Since the next Dicoogle version is intended to be a patch version, let's keep this PR on hold until we plan to deploy a feature release.

[Enet4]

@bastiao Next version of Dicoogle should be 3.4.0, so we're ready to start merging this and the other ones in the milestone:

• #660
• #598
• #695
• Maybe even #631