L-BFGS-B v2.0.0

• Removed Function template parameter from both Solver and Lbfgsb, passing a reference to the concrete Function throughout Solver. Note: C++17 is required if you don't want to have to write Lbfgsb<>.
• Renamed and repurposed SolverState::reset to SolverState::stalled, which means the solver produced the exact same iterate.
• Removed Solver::abort(). Now Solver::updateState() determines if aborting is appropriate based on the solver being stalled twice in a row.
• Along with the above changes, Solver::reset() is now only responsible for resetting the algorithm's (derived Solver's) internal data.
• Updated unit tests to work with new changes.
• Updated readme.