Add documentation about adding new algorithms

[jatkinson1000]

Closes #55

Adds basic documentation on:

• Code structure
• Data formats
• Packaging
• Testing

This is intended to provide a starting point that can be added to as users go through the experience of adding algorithms.

[jatkinson1000]

This will need overhauling now that I have opened #69 to provide Abstract Base Classes for the TCTracker and TCTrackerParameters. These provide a skeleton from which new algorithms can be added, and enforce implementation of certain methods to ensure a consistency between trackers.

I'll aim to re-write this when I get some time, but @admg26 it would be good to have you cast and eye over #69 to check that you're happy with this approach.

[admg26]

@jatkinson1000 I have looked at the refactored code and am generally happy with this.

[admg26]

This all looks good.

[jatkinson1000]

I have rebased on top of main and updated to reflect the changes to using base classes.

I believe this is ready for review in the new year @admg26 and @sjavis

[sjavis]

The documentation looks good - I can't think of anything missing. I have a few suggestions for improving the clarity.

[sjavis]

Thanks for the changes. It all looks good to me.