Initial value for aggregations

[asavinov]

Problem: currently it is not possible to set initial value for aggregations (a default value is always used). For example, if we use sum for aggregation then default initial value 0.0 is meaningful. But if we use product, then we would want to specify 1.0 as an initial value.

Implement a possibility to specify custom initial value to be used in aggregation operations like rolling aggregation or normal aggregation.

Notes:

• Consider also its relation to fillna. We need some value if the group is empty.
• It should work for both API and Column-SQL
• Create or modify unit test for initial values
• Modify notebooks and documentation to demonstrate the use of initial values