Write the initial population size of a daughter population in a summary

[bodkan]

A minor thing, but while preparing a popgen lecture I just noticed that typing out a slendr population object which involves a population that appears as a daughter population in a split (as opposed to an ancestral population) does not print an initial N of that population in the summary:

> pop5

slendr 'population' object 
-------------------------- 
name: pop5 
non-spatial population
stays until the end of the simulation

population history overview:
  - time 600: split from pop4             <---------- right here
  - time 900: resize from 100 to 50 individuals
  - time 1200: resize from 50 to 250 individuals
  - time 1600-2200: exponential resize from 250 to 1000 individuals
  - time 2400: resize from 1000 to 400 individuals

This is an oversight that is easy to fix.