Auxiliary methods for integrators

Dear all,

some of you (@jacobwilliams and @zbeekman) have expressed the idea that the FOODiE's integrators should provide some _auxiliary_ methods:

> It would be wonderful to have an object oriented library of time integrators that could be used, for integration of abstract data type (ADT) calculus (See Rouson et al.) with procedures to query their properties, like their stability locus plots sigma roots, etc. Users could then be guided to picking a good time integrator for their PDE/ODE of interest and it would take a lot of the headache out of picking a suitable time integration scheme to match your spatial scheme when making a PDE solver.

I am now quite confident with the ADT of Rouson (Euler and explicit TVD RK schemes have been implemented) thus, in the meanwhile other schemes are under development, I would like to implement some of the auxiliary methods that you asked for. Here is list tracking issue:
- [ ] stability locus _plot_;
- [ ] _event finding_... @jacobwilliams what do you mean?
- [x] supported stages/steps inquiring methods, e.g. `tvd_rk%is_supported(s=7)`;
- [x] lower and upper bound limits of supported stages/steps, e.g. `tvd_rk%max_stages()`;
- [ ] provide a _descriptive summary_ (varying string?) of the solver or of the class of solvers, e.g. `tvd_rk%description()`;
- [ ] ...