download: while running within dandiset should take that dandiset as default to download from

[yarikoptic]

So that instead of

lena:/tmp/000030
$> dandi download
Error: No URLs were provided.  Cannot download anything

the result operation should be analogous to

cd ..
$> dandi download http://localhost:8092/#/dandiset/5f282afffb242e375f756695
2020-08-03 11:41:09,963 [    INFO] Traversing remote dandisets (000030) recursively
PATH                                         SIZE      DONE    DONE% CHECKSUM STATUS    MESSAGE             
dandiset.yaml                                                                 skipped   already exists      
sub-anm369962/sub-anm369962_ses-20170309.nwb                                  skipped   same time and size  
Summary:                                     0 Bytes   0 Bytes                2 skipped 1 already exists    
                                             +796.9 kB 0.00%                            1 same time and size

the problem is that download deduces which instance (global or some local "docker") to talk to from the URL provided. So I guess the only way to quickly achieve desired ATM would be to add option already existing in upload:

$> DANDI_DEVEL=1 dandi upload --help                                                     
Usage: dandi upload [OPTIONS] [PATHS]...
...
  -i, --dandi-instance [dandi|local-docker|local-girder-only]
                                  For development: DANDI instance to use

so it could be then DANDI_DEVEL=1 dandi download -i local-docker [URLs] , and if instance specification conflicts with the one from URLs - raise an exception