External seed for metadynamics

[foxtran]

For reproducibility, it would be nice to have a way to set external seed. At the current moment, only quite strange procedure initrand exists: https://github.com/grimme-lab/xtb/blob/04ad2f85c501df22c555749200791e13d5f8883b/src/setparam.f90#L535-L540 which allows to use the same seed between all runs without an ability to change it without recompilation.