Add validator for strict non negative distribution definition

[bruno-f-cruz]

This could be implemented at the level of the distributions module by adding a functional validator that takes a distributions.Distribution generic interface and check if the truncation parameters are consistent with a strictly positive distribution. This would add the annoyance of users having to define the truncation even if the distribution does not allow for negative values to be theoretically drawn, but it would at least protect against undefined domains (e.g. negative timespans)