Allow for using prior variance and identity matrix as jumping distribution

Takes up #509
The option is now not under a different mode_compute but requires ``` mode_compute = 0 ```. Still to be added is the preprocessor token ```MCMC_start```. Allowed strings are ```prior_variance``` and ``` identity_matrix```. Open question: Should ```MCMC_start``` automatically trigger ``` mode_compute = 0 ``` or should the user haver to set this?