Fix verdi export create when specifying computers

[sphuber]

Fixes #1033

Only passing computers to verdi export create was not supported by export and hence broken.
I added the necessary logic to make computers exportable.

Additionally, I fixed the parsing of the verdi export create command.
Click does support multiple values for an option through the multiple keyword
of the option constructor, however, this requires the user to repeat the flag
for each value, i.e.:

--nodes 1 --nodes 2 --nodes 15

For big lists, this is impractical and untenable. Also it is not very intuitive and users were using it the way below, which would throw cryptic errors. Therefore we introduce a custom
MultiValueOption that instead supports the following notation:

--nodes 1 2 15

Since these options are greedy, they can clash with cli commands that also take
arguments. If the greedy option is used, the argument will be mistaken for another
option value. In this case, the MultiValueOption respects the endopts marker '--'
that is the standard for POSIX command line interfaces. Anything after this marker
will be considered as arguments by the parser.

[codecov-io]

Codecov Report

Merging #1448 into workflows will increase coverage by 0.06%.
The diff coverage is 45.61%.

@@              Coverage Diff              @@
##           workflows    #1448      +/-   ##
=============================================
+ Coverage      56.45%   56.51%   +0.06%     
=============================================
  Files            270      270              
  Lines          33517    33568      +51     
=============================================
+ Hits           18922    18971      +49     
- Misses         14595    14597       +2

Impacted Files	Coverage Δ
aiida/utils/cli/options.py	46.37% <37.5%> (+46.37%)	⬆️
aiida/orm/importexport.py	78.98% <54.54%> (-0.3%)	⬇️
aiida/cmdline/commands/exportfile.py	11.36% <83.33%> (+0.33%)	⬆️
aiida/utils/cli/validators.py	10% <0%> (+10%)	⬆️
aiida/utils/cli/__init__.py	37.5% <0%> (+37.5%)	⬆️

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