`Splitter` unequal group sizes split

[xandaau]

Current Splitter functionality allows one to generate n groups of equal size m from some dataframe using different split methods.

But several types of experiments require split with unequal group sizes. It would be nice to implement this feature in the Splitter tools. As one of the options, group sizes could be controlled using the modified groups_size parameter (from run method) of length n with different values for group sizes inside, for example: [1000, 100, 100]. If this parameter is equal to single number, the split will be made into groups of the same size.

In the future, this feature can also be used in the Designer empirical design of unequal sized groups, if we decide to integrate the Splitter into Designer methods #16.