Shortcut for computing inbreeding when creating/loading pedigree

[wintermind]

Right now, the only way to do this is using the form_nrm keyword. That's fine for small pedigrees, and really slow for large ones. It would be nice if you could tell PyPedal to load the pedigree, and as part of the loading process compute the CoI and store them in the pedigree. Note that it is important that the method for computing inbreeding be user-specifiable.

The downside is that this will require some work, and will save only one line of code for the user.