[RFC][WIP] Add more query functions into MDL

[TsumiNa]

We need to add more querying functions to MDL class. At here, I want to collect some suggestions for what function should be be given priority.

• [x] list_properties; return a list of all properties' name.
• [x] query_properties; return a list of matching property.
• [x] list_mode_sets; return a list of all model sets' name.
• [x] query_mode_sets; return a list of matching model set.
• [x] pull_mode_set; return the details of a model set and download the models in it.
• [x] list_descriptors; return a list of all descriptors' name.
• [x] query_descriptors; return a list of matching descriptor.