`initPopulation` was a heavily overloaded parameter, with a variety of
types corresponding to a range of possible behaviours. This commit:
- Removes the option to pass a vector representing the search space.
This is specific and unclear, and behaviour can easily be easily
represented by one of the other options
- Restricts `initPopulation` to be a vector of individuals (where an
individual is a single member of the population)
- Adds a new parameter `creation` to represent a function to create an
individual.

Default behaviour has not been changed, but is now represented by a
creation function.

assertion

`initPopulation` was a heavily overloaded parameter, with a variety of
types corresponding to a range of possible behaviours. This commit:
- Removes the option to pass a vector representing the search space.
This is specific and unclear, and behaviour can easily be easily
represented by one of the other options
- Restricts `initPopulation` to be a vector of individuals (where an
individual is a single member of the population)
- Adds a new parameter `creation` to represent a function to create an
individual.

Default behaviour has not been changed, but is now represented by a
creation function.

The `getIndividual` function was performing unnecessary work, so is
factored out in this commit. Instead, a sample individual is passed
as a parameter. This has the added benefit of making it clear to a
reader what kind of data is being worked with.

Note: n-queens P increased to 200 due to flaky test.

of accessing members

of accessing members

members