set COG and calc moments of inertia

[ryancoe]

Allows user to set the location of the center of gravity (gravity_center) and use this for calculating the moments of inertia.

For example

import meshmagick.mmio as mmio
from meshmagick.mesh import Mesh
from math import pi, fabs
import pytest

# analytic solution
vertices, faces = mmio.load_VTP('meshmagick/tests/data/Cylinder.vtp')
cylinder = Mesh(vertices, faces)
xmin, xmax, ymin, ymax, zmin, zmax = cylinder.axis_aligned_bbox
R = (xmax-xmin)/2.
h = zmax-zmin
V = pi*R**2*h
Ixx = Iyy = V*(R**2+h**2/3)/4
Izz = V*R**2/2

# parallel axis theorem to find MOI at base of cylinder
Ixx_base = Ixx + V*(h/2)**2             

# use meshmagick to find moments of inertia
inertia = cylinder.eval_plain_mesh_inertias(rho_medium=1.)

# shift calculation to new cog (at base of cylinder)
inertia.set_cog((0,0,-h/2))
inertia.shift_at_cog()

assert pytest.approx(Ixx_base, rel=1e-1) == inertia.xx

[ryancoe]

All tests passed

$ pytest meshmagick/tests -q --disable-pytest-warnings
..............................                                                                                                                                       [100%]
30 passed, 3408 warnings in 13.29s

[frongere]

Hi Ryan,

Thank you for this PR! That was indeed needed. And my apologies for the late action (quite busy)
For your information, I worked the last few days on a far much better implementation for the hydrostatics module. Now data of the hydrostatics report are expressed in the initial vessel frame, not in a frame obtained after successive iterations to the Newton-Raphson solver. The hydrostatic stiffness matrix is well expressed with respect to COG/buoyancy centre and no more with respect to a frame origin.

BR