Add alternative separators for element strings of the remaining functions in CellService

[tobiaskapser]

Hey,

In addition to #801 I added the alternative separators for the remaining functions to keep the interface consistent.

The rule and feeder checking functions have the possibility to pass an Iterator instead of a string. That could also make sense for the get_value function and would make string splitting obsolete. Maybe we could pass some kind of data structure that allows for the definition of multiple hierarchies too.

Best regards
Tobias

[MariusWirtz]

Thanks for the PR @tobiaskapser. I will merge it as it is now.

I agree it would be nice if the function could consume an Iterable of strings.
In that case, if a string is passed it would treat it as the special element string.
If any other type of iterable is passed the function could treat it accordingly.

As you already pointed out, a simple iterable couldn't be used to describe cells on multiple alternate hierarchies.
I wonder if perhaps we could accept an iterable of mdxpy.Member

[
  Member.of('d1', 'd1', 'e1'),
  Member.of('d2', 'h1', 'c4'),
  Member.of('d2', 'h2', 'c8'),
]