Introduce Dataset inheritance from blosc2.Operand

[lshaw8317]

Implementing a solution for #271. In the end a context manager is overkill. One can now operate on Dataset instances and mix them with blosc2.LazyExpr instances like so

    # test streamlined API
    a = auth_client.get(oppt)
    ls = blosc2.lazyexpr(f"linspace(0,1, {a.shape[0]})")
    mylazyexpr = a + 0 + ls

and then send the lazy expression to the server like so lxpath = auth_client.upload_lazyexpr(lxpath, mylazyexpr).

[FrancescAlted]

I have a couple of comments.