REFACTOR: Standardize distributions parameters naming

[iraedeus]

Currently, all parameters are in the form PARAM_*, but I don't see the point in doing so. I also need to decide whether to write the parameters in Scipy style (loc, scale, shape) or name them according to PDF formulas, etc.

[MaxPastukhov15]

I suggest naming the parameters according to the PDF formulas. From my point of view, it is more convenient.