Define estimation return types as library types

[nahueespinosa]

Feature description

Since #275, we have some estimation algorithms (estimate_clusters) that return local structs for convenience instead of creating tuples of values. We should make those types first-class citizens in the library.

Context: #275 (comment)

Implementation considerations

They should perhaps be specialized for different types of states.
It may be a good opportunity to revisit some algorithm implementations and apply the law of useful return:

A procedure should return all the potentially useful information it computed.
― Alexander A. Stepanov, From Mathematics to Generic Programming

See for reference https://en.cppreference.com/w/cpp/algorithm/ranges#Return_types.