number_of_leaves may return -1 but is used in a log function

[guimarqu]

Describe the bug
The method number_of_leaves (branchinggroup.jl:74) may return -1 but its output is used in a log function without any checking.

See score = - log(number_of_leaves(parent_delta, deltas)) / log(length(deltas)) (branchinggroup.jl:147)

To Reproduce
See the tests of PR #241

[rrsadykov]

Can you change branchinggroup.jl:147 to

numleaves = number_of_leaves(parent_delta, deltas)
if numleaves < 0
    score = -Inf
else
    score = - log(numleaves) / log(length(deltas))
end