Add case() function

[ilongin]

We need to add case() function that is similar to SQLAlchemy case: https://docs.sqlalchemy.org/en/20/core/sqlelement.html#sqlalchemy.sql.expression.case

Example usage:

dc.muate(
    res=func.case((C("num") > 0, "P"), (C("num") < 0, "N"), else_="Z")
)

A in SQLAlchemy, for now it can only accept Python primitives as results like int, float, bool, str.

[shcheklein]

Could you please clarify the need a bit?