Resolve "Improvements for v0.2.0"

Closes #21 (closed)

Major changes:

• OptVarSet split out
  • Takes upper-bound/lower-bound functionality, also replacing defaults
  • Use metadata for ub/lb
• SysVarSet uses ext_type for extended type to indicate if traj, batch, or trajbatch instead of doing inheritance. Still gives tree_map errors if you use a mix of types, but gives native pickling b/c no new object types are dynamically created
  • Helper functions on sysvarset for dimensions

What's not done: casadi_tools, solvers, see #34 and #33 (closed).