Expand covariance matrices when reading in ISIS control networks

ISIS control networks store covariance matrices in compressed format, just one triangle, because they are symmetric matrices. Here's the actual storage order, numbers indicate index in the storage array and "X" indicates absent:

```
0 1 2
X 3 4
X X 5
```

It would be very helpful if we unpacked these matrices on ingestion and then repacked them on write. Currently, anything that wants to work with this data has to unpack them each time and anything that wants to update them has to repack the updates.

No files, tests, or entry points are named in the issue. Trace control-network ingestion and write paths, confirm the triangular storage order shown in the issue, and verify that callers receive expanded symmetric matrices while serialized output is repacked.

Written by the indexing model from the issue text.