Adapt data-structure(s) & API signature(s) of Karabo

[Lukas113]

According to my knowledge, we could improve some data-structures & API's of Karabo:

• Define common API-terminology used for variable-names through Karabo in a document and apply this on the repo.
• Change all phase_center args to tuple[float, float] and unify signatures (e.g. not take ra-dec as separate args anywhere where both are needed). The reason is simply because a tuple can have a fixed length, where a list (or any Sequence) is not defined as strictly.
• Replace evil mutable-default args (e.g. SkyModel.setup_default_wcs phase_center).
• Add more kw-only args, especially to avoid boolean-trap anti-pattern. Usually, the first argument(s) are data and/or path args, which are not necessarily kw-only args. However, customization args for that particular data should be (as far as I know) kw-only to improve readability & maintainability (usually helps to be backward compatible) of code.
• Introduce unit-suffix to unit-args if units are not base SI-unit, e.g. deg, arcsec, mhz, etc. if it's not a astropy.units.quantity.Quantity, but just int or float.
• Adapt typing to accept also e.g. Sequence and not only list and/or tuple wherever it makes sense. This is also true for some other collections.abc types.
• Support str and pathlib.Path as path input for user-api args. Our API currently is not consistent in this domain.
• I'm sure there are other cases (e.g. list should be replaced with tuple to improve the data-structures and signatures of Karabo's API.
• Probably the biggest change is to think about a Public API to get to v1.0.0. Expose each function/class/module which are public and prefix-underscore each non public function/class/module. This may also include some API changes itself. However, if this is a change which is too large, it can be done in a different PR. This would contribute to issue Karabo Versioning #614 .

[Lukas113]

This shouldn't take too long I think. However, this will most likely lead to API-breaking changes.