`Splitter` multigroup fractional split feature

[xandaau]

The current fractional feature of the Splitter class only supports splitting into two groups. In some tasks, it is necessary to make a multigroup partition of a given table. It would be nice if we extended our functionality with such a function. I think it will be convenient for users to control the division of fractions between groups using the analogue of the part_of_table parameter, but in the form of a list/iterator: [0.5, 0.1, 0.1, 0.1, 0.1, 0.1].