Fix atcavityoff

[lfarv]

atcavityoff did not correctly return the index of cavities (see #446). This is corrected here.

[lfarv]

Note that there is a difference of meaning of cavitiesIndex between atradon/atradoff and atcavityoff.

• in atradon/atradoff, the cavities index points to active cavities. Since both functions allow the modification of some elements while keeping others intact, this is a valuable information,
• in atcavityon/atcavityoff (original version), the cavities index points to all cavities, independently of their state. This behaviour is restored here. The problem might even have occurred in atcavityon if the user for some reason, would set cavipass to a non-cavity passmethod.

[lnadolski]

That makes the job
Thanks