Errors of mee2rv

[gaowutong]

3 Bugs are reported about the method mee2rv.

In

poliastro/src/poliastro/core/elements.py

Lines 530 to 531 in 55e9643

	rx = (r / s2)(np.cos(L) + alpha2**2 * np.cos(L) + 2 * h * k * np.sin(L))
	ry = (r / s2)(np.sin(L) - alpha2**2 * np.sin(L) + 2 * h * k * np.cos(L))

1. Missing multiplication symbol after (r / s2).
2. alpha2**2 should be alpha2.

In

poliastro/src/poliastro/core/elements.py

Lines 534 to 545 in 55e9643

	vx = (
	(-1 / s2)
	* (np.sqrt(k / p))
	* (
	np.sin(L)
	+ alpha2 * np.sin(L)
	- 2 * h * k * np.cos(L)
	+ g
	- 2 * f * h * k
	+ alpha2 * g
	)
	)

3. np.sqrt(k / p) should be np.sqrt(mu / p) where mu is the gravitational constant. The variable k causes ambiguity, as the fifth modified equinoctial element is also k. Same for the code about vy and vz