Allow masking when visualising and analysing PAE matrices

When working with multiple PAE matrices involving the same sequence, but dramatically different numbers of row/columns (e.g. looking at alternative complex forms, p/p ia, etc), it should be possible to view relevant segments at the same scale so they can more easily be visually compared.
- Optionally hide/show rows in the matrix plot according to whether they are visible in the alignment.
----> potentially consider doing this dynamically based on current visible region, allowing quick focus on local interactions.

It may also be relevant to mask the PAE data and recalculate any trees/clusterings, or allow those submatrix clusterings to be compared to the global clustering.