Accept `<:Metric`

[Datseris]

Instead of forcing all users of VPTrees to change a metric to a function, perhaps it is much easier to use multiple dispatch in this package only once, to allow both Function as well as Metric to be the input to VPTree. For example you can internally define get_distance or so like this:

get_distance(m::Metric, a, b) = evaluate(m, a, b)
get_distance(f::Function, a, b) = f(a, b)

This happens only once on the developers end, instead of an unlimited amount of times on the user end.

[altre]

Signature is now changed to accept VPTree(data::Vector{InputType}, metric::Any; threaded=false), where metric can be any callable. This then includes Function and Metric, since StringDistances.jl and Distances.jl now both implement all evaluate as callables.