v0.99.6

New features

• add citations of books and papers used during development
• add example solving Muskat's Material Balance Equation. It uses ODESolverFactory, all ODE solvers and two nested lapply functions.
• new vignette ErrorEuler that uses an internal accumulator for the step counter. Log-log plots to show error vs solver.
• add ODE solver applications (those ending in app) to demo folder. Now demos can be run from the console directly.
• improving vignette Reaction showing more plots.
• add constructors for ODE, ODESolver and ODESolverFactory.
• using method getState instead of solver@ode@state.
• help has been reorganized to look neater and show methods with their own classes in the pkgdown website.