Restructuring in IO

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What has been done:

• Replaced unformatted sequential files by stream files.
• Sequential_file has been replaced with formatted_read_file_class. The name is not great, but it cannot be named input file.
• More functionality (like open, close, rewind, error handling) in abstract_file, basically only specific read and write routines remain in the subclasses
• format (formatted, unformatted), action (read, write, readwrite) and access (sequential, stream) are now predefined in the file classes.
• Added some routines for error handling
• MR seems large, but the actual changes are located in the io directory

Checklist

Check the boxes below after you have set up the merge request

• Versioning and website
  • Changelog is up to date
  • The patch number in parameters.F90 is correct
  • Updates (if any) to the website have been added
• Testing
  • Tested with -rcheck
  • Tested with -fbatch
• Compliance with style guide
  • Do's and dont's
  • Documentation style