Re-implement KS test in sqlalchemy expression language API

[YYYasin19]

Hey,

to track this task, I'd like to open a new issue for the future.
Currently, the Kolmogorov Smirnov test has been implemented in pure SQL, which was already a big win (cf. #28 ).

To get even more independence from different database internals and remove some of the clunky implementation details (e.g. string replacements in mssql table references to fit into the SQL template) it would be helpful to convert the SQL query into sqlalchemy expression language.

This shouldn't be too hard, since:

• the expression language API has been used throughout the project as well, so there is a lot of referene
• the test consists mostly of with-query steps and commonly-available functions
• this is completely seperated from the rest of the project (hence the good first issue label) and consists of "just" translating an SQL query into sqlalchemy expressions

Thanks in advance for whoever might be participating (even if it's just future me).

[jonashaag]

Thanks in advance for whoever might be participating (even if it's just future me).