Add a posibility to export the whole AiiDA database

[yakutovicha]

Currently there is now such an option to export the whole database.

> verdi export create --help


Usage: verdi export create [OPTIONS] OUTPUT_FILE

  Export various entities, such as Codes, Computers, Groups and Nodes, to an
  archive file for backup or sharing purposes.

Options:
  -X, --codes CODE...             One or multiple codes identified by their
                                  ID, UUID or label.
  -Y, --computers COMPUTER...     One or multiple computers identified by
                                  their ID, UUID or label.
  -G, --groups GROUP...           One or multiple groups identified by their
                                  ID, UUID or name.
  -N, --nodes NODE...             One or multiple nodes identified by their ID
                                  or UUID.
  -F, --archive-format [zip|zip-uncompressed|tar.gz]
                                  The format of the archive file.  [default:
                                  zip]
  -f, --force                     overwrite output file if it already exists
  --input-forward / --no-input-forward
                                  Follow forward INPUT links (recursively)
                                  when calculating the node set to export.
                                  [default: False]
  --create-reversed / --no-create-reversed
                                  Follow reverse CREATE links (recursively)
                                  when calculating the node set to export.
                                  [default: True]
  --return-reversed / --no-return-reversed
                                  Follow reverse RETURN links (recursively)
                                  when calculating the node set to export.
                                  [default: False]
  --call-reversed / --no-call-reversed
                                  Follow reverse CALL links (recursively) when
                                  calculating the node set to export.
                                  [default: False]
  -h, --help                      Show this message and exit.

I think it would be a very useful thing to have.

[yakutovicha]

Duplicate of the issue #974