Add support for providing XCSF with a fixed seed

[dpaetzel]

Hi!

There are two main reasons for this PR:

• When submiting many XCSF runs to a cluster, the current implementation leads to most of them starting with the same random seed (due to the dependence on the current time in rand_init).
• Fixing the random seed is necessary for repeatability.

Note that I haven't used C for quite some time now and thus may have made mistakes; however, for me, the code of this PR seems to work–so far. 🙂

[rpreen]

Thanks for this. I can confirm it works here, too.

[dpaetzel]

Great, thanks for merging!