InfostateTree python bind

[maichmueller]

This PR would bring bindings for the infostate tree class to the python module pyspiel.

The following classes would be bound by this PR:

InfostateTree
InfostateTreeNode
InfostateNodeType (enum)

TreeplexVector< double >
LeafVector< double >
DecisionVector< double>

TreeplexVector< py::object >
LeafVector< py::object >
DecisionVector< py::object >

SequenceId
DecisionId
LeafId

Range< SequenceId >
Range< DecisionId >
Range < LeafId >

kUndefinedDecisionId (constant)
kUndefinedLeafId (constant)
kUndefinedSequenceId (constant)
kDummyRootNodeInfostate (constant)
kFillerInfostate (constant)

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[maichmueller]

I have been unsure with how to bind the InfostateNode class correctly, since it doesn't have a public constructor and is constructed only by the tree. Since the tree exposes the nodes to the client, I exported the node class as well. I opted to go for a std unique ptr mock as smart holder which does not manage the lifetime but acts like one (yet remains being copyable).

Edit: the python bindings are correctly recognized, it was an error in my env setup.