Make cutoff values more usable

[fabrice-rossi]

Values computed by the cutoff.* functions correspond directly to the thresholds used in the pruning phase of VLMC or coVLMC. As the pruning tests are written as follows

if (p_value > cutoff) {
  prune
} else {
  do not prune
} 

using directly the cutoff values may not induce pruning.

This can be fixed by either:

1. replacing the test by p_value >= cutoff
2. slightly decreasing the thresholds reported by cutoff

The first solution departs from what is used in other VLMC implementations, leading to potential differences between the results of very similar codes. The second solution is easy to implement in Rcpp using the nextafter standard function.

[fabrice-rossi]

Fixed using the second solution by commits 56f7b9f and f5e280a.