Improve printing/display of `XSystem` parameters

[TorkelE]

Currently, if we have a XProblem:

using Catalyst

@parameters p d
@variables t X(t)
D = Differential(t)
eq = D(X) ~ p - d*X
@mtkbuild osys = ODESystem([eq], t)

u0 = [X => 1.0]
ps = [p => 2.0, d => 0.5]
oprob = ODEProblem(osys, u0, (0.0, 1.), ps)

we can look at its parameters using

println(oprob.p) # Gives [2.0, 0.5]
println(oprob.ps) # Gives a very long structure.

However, neither of these are very useful. The first depends on order to tell us which parameter is which (which we should not do since I don't know when). The second doesn't tell us any information at all.

Wouldn't it make sense to at least for one of these display something like

p => 2.0
d => 0.5

This creates an easy way for the user what parameter values they have in a problem.

It would be good to have something similar for u. However, I realise oprob.u0 might just be a normal vector, in which case this might not be possible. However, at least for oprob.ps this should be possible.