API: `registry` is broken

[Desiment]

Context: idea was that user can get access to estimation procedures via a special object, called register, to avoid lots of imports.

But right now, the register is completely broken due to some strange cyclical import problem. We should figure out, do we really need registry or some kind of dispatching system

• If we don't need it, we should remove it
• If we need it, than it must be fixed

[Desiment]

Current approach also discourages us to chaining estimators (e.g. estimate sigma in NM-mixture, then estimate density) or using "smart" strategies for estimation of densities (e.g. use one approach to estimate density on [0;1] and other approach for values from (1; +inf))