adds datastructure for populations to nodes

- adds population variable to nodes
- adapts buildInitialTree to create the nodes with population set correctly
- adds a unit test for buildInitialTree
updates unit test
makes time interval track pop numbers in contemp.
fixes wrong coalescence rate in special cases
firest step adding populations to sampling contmp.
changes calculation of rates to reflect pop.struct
structure maybe working, migration missing
adds model infrastructure for mirgration
started including mig & growth in  coalescence
adds 'events' and starts to use them
adds test for event type sampling
adds more tests and fixes bugs for event sampling
further unitized code
finishes refactoring of coalescence function
adds treebuilding with migration

+ but there is still a bug with sig sites generation
migration seems to work
Merge branch 'pop-structure'

Conflicts:
	src/forest.cc
modifies pruning to exclude migration nodes