[storescu|echoscu] Compact way to set called AE title

[Enet4]

As a user of these tools, I would like to specify the called application entity title alongside the AE host and address, making it compact and easier to remember.

Instead of this:

dicom-storescu --called-ae-title PACS-ARCHIVE localhost:104

I would be able to write:

dicom-storescu PACS-ARCHIVE@localhost:104

[Almeida-a]

I believe this would require a change of the dicom-storescu syntax from:

dicom-storescu [FLAGS] [OPTIONS] <addr> [files]...

to:

dicom-storescu [FLAGS] [OPTIONS] <called-ae-title>@<addr> | <addr> [files]...

And the equivalent for dicom echoscu (without the files tokens). We would need also to keep backwards compatibility with older versions.