make io an interface

[Kwasniok]

Let all data types which support IO inherit from a new abstract IO interface supporting write and read(parse_node).

PRO:

• proper object orientated approach CON:
• increased dependencies since most data types do not make use of virtual functions/class hierarchies

The current sate is inconclusive.

[Kwasniok]

revised approach:

Use template function for read and write (similar to operator<< & operator>>).