Feature solo_mgar

[dietmarwo]

This UDP represents some of the objectives of the original solar orbiter mission:
- ballistic transfers
- multiple GA manoevers at Venus and Earth
- limited mission time
- inclination > 33 deg at the end of the mission
- low perhelion near 0.28 AU during the EMP phase.
thereby fulfilling its constraints.
To keep it as simple as possible, this UDP currently works with the fixed original planet sequence EVVEVVVVVV,
more flexible variants may be created later. Each EV / VE transfer is performed as MGA,
each VV transfer is performed as resonance. This means, for a free beta parameter the corresponding orbiter
period is computed. From the given resonance alternatives the one is chosen, which minimizes the timing error
- how much earlier / later than venus the orbiter arrives after the resonance transfer.
Instead of "caching" of position / velocity data 'rvt' objects are used: Orbit representations
storing position, velocity, time and mu.
The UDP provides a hand crafted 'unconstrain' activated if use_constraints == False which was used to compute the
reference results at https://github.com/dietmarwo/fast-cma-es/blob/master/examples/data/solo_results.txt
An example for test/optimize is here: https://gist.github.com/dietmarwo/5b6bbadbd696d36806177be2df3c6bb7

[dietmarwo]

Updated solutions to compare all three solar orbiter models are at

https://gist.github.com/dietmarwo/86f24e1b9a702e18615b767e226e883f

solodsmtest.py - for pykep.trajopt.gym._solar_orbiter._solar_orbiter_udp_1dsm
solotest.py - for pykep.trajopt.gym._solar_orbiter._solar_orbiter_udp
solomgartest.py - for pykep.trajopt.gym._solar_orbiter._solo_mgar_udp