implement viability selection
implement selecting parents
make Population 2nd argument
move abstract type Sex and its subtypes into a separate file.
bring Parameter more akin to Marcy's
delete unused file.
get rid of reference to a deleted file.
add a new Population constructor, which leaves the most of fields uninitialized.
start working on piecing everthing together.
set fitness as soon as migrate or trait is set.
slight cleanup.
coeff of sexual selection is computed outside of the parent chooser.
WIP: simulator.
add a list of differences between the model and implementation.
fix Markdown syntax
each offspring gets a new Lineage node for each locus.
implement top-level functions