Remove mandatory argument for Reader/Engine

[magnusuMET]

This is to make nesting of pyaro configurations easier. This is a breaking change and will affect pyaro-readers and pyaerocom

[heikoklein]

I don't understand why you want to remove the readers handler as mandatory argument? It makes it very clear that the reader only concerns the data-format, while the datasource must be in a different place.

You also change the handler (filename_or_obj_or_url) to a filename. Not all readers will implement a disk-base filename, but rather a url (ACTRIS/EBAS).

There is no issue related to this PR, so I don't see any why.

[magnusuMET]

There is a linked PR which should explain a bit more why I would like this change. It essence it allows serialising the arguments which the positional argument don't allow.

This is still a draft, I am working with @dulte to hash out some ideas (this being one of them)

[heikoklein]

Looking at the PR in pyaro_readers, this looks to me like datasets is the handler her and just needs to be renamed to filename_or_obj_or_url?

[magnusuMET]

Not all datasets requires a filename_or_obj_or_url, such as the data-merger (or post-processor). The first argument of the reader is not required to be named filename_or_obj_or_url which means it has to be unpacked and given as a positional argument