FIX on attelo decode when no relations are supplied

I wish we had something like the Maybe monad
Raise exception when user asks for too many folds

Closes #7

Alternatively, we could have silently generated the min
of the two, but I suspect this would have knock-on
consequences for test harnesses that may not themselves
have anticipated the problem. The worst would be if it
silently does the wrong thing.

At least this way we can more quickly identify the problem
Also require at least 2 fold evaluation
FIX: crash on edge case where nothing is attached
Remove astar heuristics tests

Redundant with tests moved to attelo.decoding
Simplify astar heuristic an Enum

This is less flexible but a lot cleaner than wrapping
a function and associating a name with it.

Fixes #10
Remove likely unused report-saving code