v0.8.4: Improved variable name tracking

• updated readthedocs configuration
• updates to ODESystem.extract and the plotting functions that use it: Users can now flexibly switch between using the parameter/variable indices when specifying the variables to plot, or whether they want to use the pyrates-style naming scheme
• added option to ODESystem.run to store only the minimum and maximum of each selected state variable for periodic solutions (set keyword argument reduce_limit_cycle=True)
• debugged scenario where initial condition was run, but not explicitly labeled, which caused issues with extracting starting points from the solution branch
• debugged issue with selecting specific variables and parameters that should be saved in the continuation summary
• added private method ODESystem._map_var that ensures stable mapping between user-provided variable names and intrinsic variable names