Move macro commands from generator method to template files.

* gen_stitching_macro_code() now requires the path prefix for reading the
  corresponding template files
* macro code is returned as a list of strings now (single string before)
* write_stitching_macro() is therefore now using writelines()
* the split template design allows ensuring all required variables are set to
  a default value as ImageJ doesn't provide methods to check if a variable
  exists or not and would just stop otherwise
* changed macro code is updated accordingly in the sample_data submodule
use "File.separator" instead of hardcoding it
Add documentation and ToDo.
Optionally adjust the path separator for the tile config.
Use 80 char horizontal rule to separate output sections.
Print overall macro runtime duration.
Macro coding style (ws only).