overload @show for Params, Vars, Problem, TimeStepper

[navidcy]

We should overload @show for Params and Vars struct.
Can we loop over the contents of a struct, e.g., Params and printout all it's content variable names?

[glwagner]

Something along the lines of

print(io, Tuple("$p: $(getproperty(params, p)\n" for p in propertynames(params))...)

might do it

[navidcy]

#192 added some show() overloads; I'm closing this.