Floating point error accumulation in contained coalescent trees

[yasuiniko]

When a coalescent tree is contained within a species tree with longest distance to root of N, the longest distances to root in the coalescent tree are slightly longer than N. I believe this can be solved by using Decimal, or at least importing some popular numerical packages so that users can convert their edge lengths to a numerical representation of their choice.

If that is not in line with the vision of the project, please change the documentation in treemodel.py to reflect the types of edge lengths.

Best,
Niko