Improve structure of validated and non validated IO

[roskakori]

Currently validator imports cid and _tools.

Current issues:

• _tools is overburdened and contains extremly basic functions and fairly high level functions *_rows() --> move *_rows() to a new module, e.g. io_tools? --> note: moving *_rows() to validator would introduce a circular dependency to cid.
• cid is an annoying module name because you cannot name a variable cid anymore; maybe we should call it cids or (yet again) interface? The latter seems ok now that eventually Cid will be imported by __init__.py.
• validator is a misnomer for a module; validation would be better but feels somewhat too long a name.

[roskakori]

@heubi95: if you rename io_tools.py and cid.py, please also rename test_io_tools.py and test_cid.py.

[roskakori]

Done.