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.