Add probs-directory

containing files which assign probabilities to functions.
Current usage: switch off functions which behave badly during
random generation, e.g. "for_every"
causes the random generator to "take for ever", so we use the 0-probability
to make the generator ignore this function.

Another possible usage: do exhaustive generation but switch off all non essential
functions to keep the output small, e.g. switch off all the generalized quantifiers
except for one because syntactically they are the same. (oops, exhaustive generation
cannot be controlled by a probability file :().