3187 add other systems to st availability model

[j-a-foster]

Description

Added unplanned unavailability calculations for systems from iavail = 2 for more comprehensive availability calculation.

Checklist

I confirm that I have completed the following checks:

• [x] I have justified any large differences in the regression tests caused by this pull request in the comments.
• [x] I have added new tests where appropriate for the changes I have made.
• [x] If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• [x] If I have made documentation changes, I have checked they render correctly.
• [x] I have added documentation for my change, if appropriate.