New design functions, checkpoints, and minor updates

[chaitjo]

This PR includes the following new additions:

• Adds all gRNAde checkpoints for various data splits benchmarked in the paper, as well as for all of RNAsolo (for general usage in new RNA design scenarios).
• Adds functionality for partial re-design where parts of a sequence are fixed and conditioned upon during decoding.
• Adds a design notebook which can be used to create designs for new structures, evaluate them through a variety of metrics, and visualise the designs interactively.