Add functions to return DVs for `pl2pl_N_impulses`

[nh2]

Currently, the pl2pl_N_impulses chromosome -> DVs computation is inlined in the pretty() function in https://github.com/esa/pykep/blob/6425e53f0efd74997a7ad6873791f355e2ae2e93/pykep/trajopt/_pl2pl_N_impulses.py#L234-L283

This makes it inconvenient to get those values out.

It would be nice to get them directly via the API, similar to https://github.com/esa/pykep/blob/6425e53f0efd74997a7ad6873791f355e2ae2e93/pykep/trajopt/_mga.py#L247