Resolve "Request for changes: Change our random number generator"

Description

Replaces the PRNG by the simplest version of xorshift.

News snippet

We have changed the pseudorandom number generator (PRNG) from a simple linear congruential generator (LCG) to a shift-register generator, namely xorshift64, as introduced in Marsaglia, G. (2003). Xorshift RNGs. Journal of Statistical Software, 8(14). This new PRNG has much higher statistical quality and a much longer period than the old LCG, which should minimize issues arising from bad random samplings.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

Closes #994 (closed)