Adding the ability to handle branching and poly-disperse systems

handling non-uniform molecules
- position generation
- what data structure are we getting for a branched polymer
- generate deltas for all particles - don't worry about having it match length of particles
- cumsum - changes depend on length and chains seperately

No file or test is named. Start by tracing the current position-generation and cumulative-sum workflow in the Jupyter Notebook, then clarify the branched-polymer data structure and particle-delta behavior. Done means branching and poly-disperse systems are handled with the required deltas and separate chain and length dependencies.

Written by the indexing model from the issue text.