Simplify importing classes/functions

[LiamPattinson]

I think it'd be a good idea to expose some classes/functions to the top-level namespace. It'd be nice if we could do stuff like:

import archeryutils as au
my_target = au.Target(...)

We'll have to be fairly selective though, as most of the internals probably don't need to be part of the user API.

[jatkinson1000]

Yep, probably easiest to arrange a meeting to discuss this at a convenient point?

[jatkinson1000]

@LiamPattinson I have implemented this over in #55 if you want to take a look and share any thoughts/review.