Pre-build structure specific node groups.

[BradyAJohnston]

Some node groups are not pre-defined, to allow for custom inputs such as a chain selection, chain color and other structure-specific nodes. This allows one chain selection node to have Chain A and Chain B as inputs, while a second chain selection node that is specific to another structure can have Chain X, Chain Y and Chain Z as inputs.

Currently the chain selection node is generated when the used goes Molecular Nodes -> Selection -> Chain. It takes the relevant information associated with the object, and generates the custom node group. Before this is done however, the node and it's possible inputs and outputs are not available to Blender for the quick search and link menu that is available when a user drags out from the input or the output of another node.

If we generated all possible selection, color and other structure-specific node groups when a structure is imported - then it would be possible to use the drag select shortcut without first having to add the node.

Below is an example of drag menu in action, which can speed up workflows significantly.

https://github.com/BradyAJohnston/MolecularNodes/assets/36021261/a1261072-18a9-45f1-b3b2-b935c022502e