refactor: Split auxiliary function into two
refactor: Split auxiliary function into two

Split auxiliary function into two: 'prob' (to compute array q) and
'bounds' (to compute array Z).

Give meaningful function names: 'aux' (q, Z) -> 'prob' (q), and
'bounds' (Z); 'aux2' (z) -> 'variates'.

federicomariamassari/willow-tree