Allow easy inclusion of sample inputs in gen/GEN

It's common to have 1 or 2 input files copied from the task statement. Often we just allow that from the generator. For example, if the seed is <= 0, then we call a python module which "greps" the (-seed)th input from the tex file.This has led to code duplication among generators and a lot of symlinks to that little python module.

This PR enables the inclusion of a local text file without having to touch the generator. Example gen/GEN:

    # N    type     seed
    #ST: 0
    #COPY: testo/example0.txt
    #COPY: testo/example1.txt
    #ST: 20
    20      0          10
    500     1          11
    ...