Extract a struct for building bitmaps

[zebmason]

First pass tidy up as seen in the video referenced in #403

[matt-gretton-dann]

I think this PR doesn't go far enough. Bitmap handling is probably one of the most easily abstracted out parts of the code.

My expectation is that the only interface that needs to be exposed to the main part of the model is:

// some enums...

class BitmapBuilder
{
public:
   BitmapBuilder(/* Init stuff */);
   void record_activity(type, position);
   void output(filename, format) const;
};

See #399 for some recent changes which I think we could take further as population/infected/recovered/treated could be converted into enums themselves.

[zebmason]

@matt-gretton-dann Just a first pass as I'm wary about trying to get changes accepted given that I've had several PRs just rotting for over a month leaving really bad merge conflicts to get resolved.