Add CountByGroupSpec, CountByPIDSpec; use enums for AggregateByXSpec

[hmgaudecker]

In #780, we introduce

@dataclass
class AggregateByGroupSpec:
    """
    A container for aggregate by group specifications.
    """

    aggr: str
    source_col: str | None = None

In order to get better typing, we should remove the None option for the source col. Way to go is a separate class for counting, which is the only case how it can become None.

Same for the PID version.

The type annotation for the aggr function currently is a string and should be an enum-style type.

[timmens]

Took care of the GroupSpec versions in #860, but not the PID versions. This is still open until then.

[hmgaudecker]

Done.