Improve update command

[svenrdz]

This improves the update command and fixes a few bugs.

Resolves https://github.com/ESGF/esgf-download/issues/5 Resolves https://github.com/ESGF/esgf-download/issues/15

Changes

• Fix error when updating queries that return 0 file
• Skip inserting a file when its file_id already exists. This is caused by different checksums, most likely a publication issue.
• Print short message for each updated query with the number of associated files
• No more print(query) before each "update?" prompt, add a show option to that prompt