Option to store light version of resolution matrix

Hi @sbailey, @julienguy, ticket following our earlier discussion.

The resolution matrix is great for redshift fitting, and for studies of the small scale Lyman alpha clustering.

However, it makes the file size significantly larger, and it would be nice to be able to store a lighter version of the files that have a simpler resolution matrix, for instance a diagonal matrix with a simple Gaussian sigma describing the resolution.

This is particularly important for Lyman alpha mocks for BAO, where we want to store at least 1000 realizations of the DESI survey, and we'd really like to have smaller file sizes.

No file, test, or entry point is named. Start by locating how the resolution matrix is stored and represented, then clarify the desired selectable light representation for Lyman-alpha mocks; done should include a diagonal Gaussian form and measurably smaller files.

Written by the indexing model from the issue text.