Review `--partialtapesnumber` tapepool field

Problem

The terminology of the tapepool field --partialtapesnumber is a source of confusion.

It should refer to the desired number of tapes, in a given tape pool, that should be eligible at any time for writing new data.

For a tape to be eligible for writing new data, it should fulfil these criteria:

• Tape in ACTIVE state.
• Tape NOT FULL.
• Library NOT DISABLED.
• Tape NOT IMPORTED (nor from CASTOR).

Proposal for discussion

• Replace --partialtapesnumber by a more appropriate parameter name.
• Replace the corresponding key in the JSON output of the cta-admin --json tapepool ls command.
• Add a new JSON field for the current number of tapes available for writing?
  • This is different from the desired number of writable tapes, and requires CTA to calculate this value on request.