Investigate and resolve warnings for trackby functions in templates for pivot grid

[dkamburov]

Investigate and resolve warnings for trackby functions in templates for pivot grid

[MayaKirova]

@dkamburov @damyanpetev

In general since the pivot config is a complex object, once you start changing it runtime all inner object refs around the configuration change, hence it causes a full re-render in the different templates.

Around the idea of choosing some unique ids for the dimensions to track them by, originally I though that the memberName can be duplicated and is not a good id, since you can have different memberFunction to extract different values for each. However it seems that actually a scenario with duplicate memberName breaks the pivot:
https://stackblitz.com/edit/fnyrjhhu

And since it doesn't work anyway, I suggest we just document that it has to be unique and use it for the trackby function.