Fix mistake in TextFileWriter::TextFileWriter(), that thrown
error_opening_input_file instead of error_opening_output_file.
Creation of class error_opening_output_file in posexcept.
TextFileWriter:
- OutputFileMedium becomes TextFileWriter.
- Method write_string_to_file() becomes write_text() and is made public.
- Add a unit test.
InputLogCSV and OutputCSV do not extend OutputFileMedium anymore, but
use a TextFileWriter as an attribute.