Add ability to load sketches into memory using python API

[johnlees]

Useful for repeated queries, as otherwise they would have to be loaded from HDF5 each time

[johnlees]

See: https://pybind11.readthedocs.io/en/stable/advanced/cast/stl.html#making-opaque-types
We can then write functions which return this type to python for finer control:

• create DB, which returns a vector of Reference
• load sketches (from a HDF5 db); returns vector of Reference
• save sketches (to a HDF5 db); takes vector of Reference
• query, which takes two vectors of Reference

The current query DB function will be rewritten as load + query
The current create DB function will be rewritten as create + save