Add discrete_genome property to TreeSequence

[jeromekelleher]

It's probably worth defining a cached property on the TreeSequence object called discrete_genome that checks if all genome coordinates are integers. Or, I guess, to do it properly, we'd add a discrete_genome bool attribute to the tsk_treeseq_t struct which checks if all genome coordinates are integers during initialisation and sets the flag accordingly. I don't think we should add it to the table collection, because then we'd have to worry about the flag going out of date, as the tables are updated.

This would help settle some other long-standing issues like #353.