Integer overflow in lattice RNG

[jc-harrison]

I have found a bug which appears for large lattices when using a fast-discard RNG.

In lib/lattice/Lattice_rng.h (lines 149-167) we have:

static void Skip(RngEngine &eng,uint64_t site)
    {
      uint64_t skip = site;
      skip = skip<<40;
      eng.discard(skip);
    }

If site >= 2^24 then skip<<40 gives an overflow, so the (2^24+n)th site gets an identical random sequence to the nth site.

[paboyle]

Patch committed to develop; underlying problem is the ".discard(uint64_t) " interface does not have
enough bits for all possible lattice calculations.

Compromised; the earlier 40 bit shift was (erroneously) conservative about the number of randoms in a quenched evolution and compromised the volume too much.

Note that 5D fields are even worse.

After our call today, we concluded 30 bits (1 billion draws per site) is enough for all except some rare quenched evolutions.

Hence I moved the develop branch to shift=30,
inserted an assert to check for overflow in the 64 bit

((site << shift)>>shift) == site