Add RNA 3D motif annotations per PDB file

[AntonPetrov]

We should provide RNA 3D motif annotations per PDB file similar to how secondary structure is included for each PDB file. This would be more correct that providing family-level motif annotations because there may not be enough evidence to support the existence of a certain 3D motif in all members of the family.

This can be done (semi-)automatically using the RNA 3D Hub API to get a list of motifs and their locations.