fix(paohvis): crash with attribute selection and mergeData toggle

This merge fixes the unexpected behavior of paohvis when selecting/deselecting attributes and when using the margeData functionality. Most of the changes occurred in the dataProcessing.tsx file where I disentangled the processing of rows/cols and added the mergeData toggle, which was only partially affecting the result. I also added functionality to the attributes dropdown to prevent users from toggling attributes like _id or computed ones (# Connections), which should preserve their default selection value.