Modify Member.getHydrostatics

[YangHaoran12]

Small corrections to Member.getHydrostatics in evaluation of cross section area/moment of inertia, and the point which Fvec acts on

Purpose

1. A more accurate method to evaluate the cross-sectional area and moment of inertia of a member clipped by the waterplane, since the clipped area is no longer a circle or the original rectangular but an ellipse or a projection of rectangular.

2. moment about A end changed to about buoyancy center of segment: r_center

Type of change

What types of change is it?
Select the appropriate type(s) that describe this PR

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• [√] Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

It seems that hydrostatic test data used for testing is not very good, the results are only tested in my own code with the aid of meshmagic (https://github.com/LHEEA/meshmagick).

Checklist

Put an x in the boxes that apply.

• I have run existing tests which pass locally with my changes
• I have added new tests or examples that prove my fix is effective or that my feature works
• I have added necessary documentation