write_control_file(): function to write the yaml control file needed by read_cross2()
A few additions to doc for read_cross2
Add to the vignettes

 - in input_files.Rmd, a bit about reading in data (the advantage of the
   control file, and the possibility to create and read a zip file)

 - in developer guide, a bit about the advantage of the control file for
   reading in data, and about write_control_file, for creating the YAML
   file.
Add note to mention write_control_file in the input_files vignette