Text diagrams extraction

Such feature only available from MadGraph5 versions higher than 3.8.0 (see pr #398 for more details)

After generating and outputting the process, use:

display diagrams_text ./diagrams/proc_name --no_open --merge

to extract and merge all the text diagrams. You might want give the merged file a different name other than all_diagrams_text.txt. You can do this inside the MadGraph5 CLI via

shell mv ./diagrams/proc_name/all_diagrams_text.txt ./diagrams/proc_name/all_diagrams_proc_name.txt

or outside the MG5 CLI using same command without the shell.