Remove serialisation from Corpus (arrow)

[rkrug]

At the moment, suthors and topics are still serialised. THis makes handling more time consuming and complex.As parquet / arrow stores complec and nested data structures as well, it would be useful to store these in these formats as well.

The problem is that openalexR::works2df() does produce inconcictent structures (logical instead of string, missing columns, etc.)

• Standardise the downloaded data structure
• change code to handle these complex and nesrted structures