Issue: Correct CB calculation for partially submerged members in `raft_member.py`

[SevenSky777]

Description

The current implementation incorrectly uses rA when computing the length of the underwater segment and the center of volume. For partially submerged members, this leads to inaccurate center of buoyancy (CB) calculations.

Changes made:

• Wrong:

LWP = abs(rA[2]/cosPhi)   # get length of segment along member axis that is underwater [m]

Corrected:

LWP = abs(rB[2]/cosPhi)

• Wrong:

r_center = rA + self.q*hc   # coordinates of center of volume of this segment in the global frame [m]

Corrected:

r_center = rB - self.q*hc