Add headers to input and output files

[cgr71ii]

The provided input file to either Bifixer or Monofixer now might contain an optional header. Optional output header. Changes:

• Option --header if the input file contains header. In that case, the options --scol, --tcol, --sdeferredcol and --tdeferredcol will be expected to be the name of the field of the header instead of the position.
• Option --output_header <header_field_1>[,<header_field_2>[...]] can be set in order to specify the output header fields. If is not set, there will not be output header. If --header is set, there will not be necessary to add the name of the fields to --output_header since the provided headers in the input file will be used in the output.
• Documentation updated according to these changes.

These changes have been tested with basic configuration of the tools and, apparently, does not add any unexpected behaviour beyond the mentioned input and output headers.

Edit: I have removed the option --output_header since I think it is not useful and because the output fields can be extracted from the input header if set. Now, if --header is set, the output header will be printed using the provided values in the header.

[cgr71ii]

Once PR #12 is accepted (if accepted), this PR will need to add changes in order to implement the logic to the new flags implemented in #12. When done, this PR will be ready for review

[cgr71ii]

I force-pushed becuase I made a commit which didn't upload half of a file, so the log history was lost. This happened due to running out of space, which was totally unexpected to me until I noticed which half of a file was missing while reviewing the diff in the commit.