Relax generic requirement on ClusterItem on clustering related interfaces

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**Is your feature request related to a problem? Please describe.**
Currently the library requires you to implement `ClusterItem` on your data classes. This can be problematic when you don't own those classes and/or producers of the data (or just don't wan't your model to depend on this particular library).

**Describe alternatives you've considered**
Creating wrapper implementing `ClusterItem`. This solution gets slow with large datasets/fast changing data. Additionaly it's just plain waste of memory.

**Describe the solution you'd like**
Remove the `extends ClusterItem` from interfaces like `ClusterRenderer` and `Algorithm` and thus in unopinionated manner enable other techniques of handling this problem.