Add slicing to ContResult

[twildi]

As one can index into ContResult, it would probably make some sense to be able to slice into it as well.

[rveltz]

Hi,

Thank you for the PR!
How did you come to this, do you find it useful?

Can you add some test to ensure the coverage please?

[twildi]

I wanted to plot only a subset of the continuation results while still benefiting from the syntactic sugar of the built-in plot methods. Something like

plot(br1[40:60], br2[5:end], vars=(:x, :y))

instead of

plot(br1.x[40:60], br1.y[40:60])
plot!(br2.x[5:end], br2.y[5:end])
scatter!([br1[sp.idx].x for sp in br1.specialpoint if sp.step in 40:60], 
    [br1[sp.idx].y for sp in br1.specialpoint if sp.step in 40:60])
...

If this is of interest, I'll play around with it some more and add some tests.

[rveltz]

Yes, it is a cool idea! If you add a test, Ill merge your PR.

[twildi]

Okay, I think it's ready to merge.

I also fixed a bug in Base.getindex(br::ContResult, k::Int) where in the case of save_eig_every_step >= 2 indexing into the ContResult would return the eigenvalues/eigenvectors for the wrong step.