Style of __repr__ in Individual

[galenseilis]

Personally, I find it more readable when the __repr__ for classes like this have brackets around the information being displayed as object attributes and tells us what type of information that it is.

I.e.

The output

Individual 1

is simpler than

Individual(id_number=1)

but explicitly shows us what attribute is being communicated.

[galenseilis]

TODO: Update tests of __repr__.