Update initialization for SysVarSet, add typecasting to tree_map_relaxed

API changes:

1. generate_traj and generate_batch are no longer class methods, but normal methods (i.e. they must be called on an instance). This is to make the initialization values more clear
2. tree_map_relaxed can now take an optional typ kwarg, and this allows direct casting to Traj, Batch, or TrajBatch. If not given, the type of the first tree argument is used.

Closes #19 (closed)