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

pykep/pykep/trajopt/_pl2pl_N_impulses.py

Lines 234 to 283 in 6425e53

	def pretty(self, x):
	# 1 - we 'decode' the chromosome recording the various deep space
	# maneuvers timing (days) in the list T
	T = list([0] * (self.N_max - 1))
	for i in range(len(T)):
	T[i] = log(x[2 + 4 * i])
	total = sum(T)
	T = [x[1] * time / total for time in T]
	# 2 - We compute the starting and ending position
	r_start, v_start = self.start.eph(epoch(x[0]))
	if self.phase_free:
	r_target, v_target = self.target.eph(epoch(x[-1]))
	else:
	r_target, v_target = self.target.eph(epoch(x[0] + x[1]))
	# 3 - We loop across inner impulses
	rsc = r_start
	vsc = v_start
	for i, time in enumerate(T[:-1]):
	theta = 2 * pi * x[3 + 4 * i]
	phi = acos(2 * x[4 + 4 * i] - 1) - pi / 2
	Vinfx = x[5 + 4 * i] * cos(phi) * cos(theta)
	Vinfy = x[5 + 4 * i] * cos(phi) * sin(theta)
	Vinfz = x[5 + 4 * i] * sin(phi)
	# We apply the (i+1)-th impulse
	vsc = [a + b for a, b in zip(vsc, [Vinfx, Vinfy, Vinfz])]
	rsc, vsc = propagate_lagrangian(
	rsc, vsc, T[i] * DAY2SEC, self.__common_mu)
	cw = (ic2par(rsc, vsc, self.start.mu_central_body)[2] > pi / 2)
	# We now compute the remaining two final impulses
	# Lambert arc to reach seq[1]
	dt = T[-1] * DAY2SEC
	l = lambert_problem(rsc, r_target, dt, self.__common_mu, cw, False)
	v_end_l = l.get_v2()[0]
	v_beg_l = l.get_v1()[0]
	DV1 = norm([a - b for a, b in zip(v_beg_l, vsc)])
	DV2 = norm([a - b for a, b in zip(v_end_l, v_target)])
	DV_others = list(x[5::4])
	DV_others.extend([DV1, DV2])
	print("Total DV (m/s): ", sum(DV_others))
	print("Dvs (m/s): ", DV_others)
	print("Tofs (days): ", T)

This makes it inconvenient to get those values out.

It would be nice to get them directly via the API, similar to

pykep/pykep/trajopt/_mga.py

Line 247 in 6425e53

def _compute_dvs(self, x: List[float]) -> Tuple[