Parser rand functionality

Added a random number generator to the parser. It introduces a new variable "RandomSeed" that initializes the random number generator. The variable "rand" returns a random value between 0 and 1 (I would prefer rand() by I haven't managed to get bison to accept this).

This is not very easy to implement, as the RNG has to be seeded before the random values are used. So RandomSeed is parsed directly by the parser library first, and not by Octopus.

Probably this change should be merged after the new release branch, as it changes the parser and might have introduced new bugs.