verdi import should automatically migrate export files

[ltalirz]

currently, when trying to import an export file that was generated with an older format, I get the error message

Critical: /.../SSSP_efficiency_pseudos.aiida cannot be imported: Export file version is 0.3, can import only version 0.4
Use 'verdi export migrate' to update this export file.

From a user perspective, most of the time you simply want the data imported, so I propose that, by default, this happens automatically (with migration in memory).
We can add an option to disable this automatic migration for people who don't like it for some reason (large export file perhaps, memory issues).

[ltalirz]

Result of joint discussion:

• migrate automatically by default but prompt for confirmation by user
• notify about migration that is going to happen
• explain which command to run otherwise (this message already exists)
• add --non-interactive option that skips prompt
• for automatic migration: go via temporary file (and delete it after migration)