Changed Beam Threshold

[CatarinaSilva]

• The older expression (before May) was comparing probabilities with logarithms, and although this was fixed with the previous commit the expression should be a sum and not a multiplication, to keep the same meaning.
• Also, multiplying the beam threshold with the log(10) factor turns the parameter more intuitive
• The default threshold also makes more sense with this expression

[tomaspinho]

@redpony comments on this are deeply appreciated.