Implementation of a function chaining mechanism

This merge request addresses the issue of function composition and chaining (see #118 (closed)).

The proposed chaining mechanism is based on the idea of flagGeneric and procGeneric and comes without additional core functionality. the relevant function (which really needs a better name, ideas are very welcome) takes an function argument of type Callable[[saqc.core.SaQC], saqc.core.SaQC] and evaluates this function on a copy of the original SaQC object. That implicates, that we simply use the existing evaluation mechanisms provided in core on a copy of the original and integrate this copy back after evaluation.

To give a more concrete idea of the implementation:

saqc = SaQC(data, flagger)
saqc.functionChain(
    fields=["z"],
    lambda s: s.flagRange("x", min=0, max=100).flagRange("y", min=0, max=10).procGeneric("z", lambda x, y: x+y)
)

In the above example a (deep) copy of data and flagger within a new SaQC object will be passed to the lambda function, the the usual SaQC business is executed and afterwards only the fields given in fields are copied from s to saqc.

A few consideration I would like to point out right from the beginning:

• The present implementation is a MVP, working, but not feature complete (although I think it would still be mergable, but that is just me...).
• The MR sit on top of !167 (merged) as the feature move proposed there also solves an issue here. But !167 (merged) is not a prerequisite.
• fields can be a collection, which is in contrast to our current policy enforcing scalar fields.
• Its maybe a bit verbose.
• There is no implementation for the configuration syntax, so this is still a TODO.
• As pointed out within the code, we loose additional arguments to the SaQC-Object due to the copy. This is unfortunate, could be resolved, but maybe things get easier, when we restructure the function interface.
• And now for the most spectacular feature 🥁 : It is simple and only uses the machinery already present! I mean, come on, it's just 10 LOCs ...

Any comments are highly appreciated @luenensc and @palmb !