Update database API

[lukasturcani]

the database API needs to support fetch of multiple molecules and values in one go, to minimize database reads and writes

# get mulitple molecules in one database read
molecules = db.get(
    keys=(
        {"inchi": "one"}, 
        {"inchi": "two"},
    ),
)
# put multiple molecules in one database write
db.put(
    molecules=( 
        stk.BuildingBlock(...),
        stk.BuildingBlock(...),
    ),
)
# get multple values
values = value_db.get(
    molecules=(
        stk.BuildingBlock(...), 
        stk.BuildingBlock(...),
    ),
)
# put multiple values
value_db.put(
     (stk.BuildingBlock(...), {"prop1": 1, "prop2": 21}),
     (stk.BuildingBlock(...), {"prop1": 2}),
)
# not sure how to deal with the current API of 
value_db.put(
    stk.BuildingBLock(...), 
    1,
)
# I might continue to support it, or I could depcrate it in favour of 
value_db.put((stk.BuildingBlock(...), 1))

ofc the single instance approach will still work, as it does in the stk api normally, however, the result will always be
returned as an iterable

molecule, = db.get({"smiles": "CC"})

However, in order to not break users, initially, if you do

result = db.get({"smiles": "CC"})

you will get a warning, that your return value will turn into an iterable in a future release, to remove the warning you should explicilty set change to

molecule, = db.get(
    keys=(
        {"smiles": "CC"}, 
    )
)

So

molecule, = db.get({"smiles": "CC"})

will always work, just the return type will change.

[lukasturcani]

I think this issue needs to keep an eye on #395