Replace `gravity` with `body force`

[keiyamamo]

gravity should be replaced with body force. Current implementation only allows us to apply force in y-direction

turtleFSI/turtleFSI/modules/solid.py

Lines 63 to 66 in c52ebc2

	if gravity is not None and mesh.geometry().dim() == 2:
	F_solid_linear -= inner(Constant((0, -gravity * rho_s)), psi)*dx_s[solid_region]
	elif gravity is not None and mesh.geometry().dim() == 3:
	F_solid_linear -= inner(Constant((0, -gravity * rho_s,0)), psi)*dx_s[solid_region]

Oasis can be used as a reference point for the body force.
https://github.com/mikaem/Oasis/blob/da712b87a8998d0669df0448b1a4bbd9fea284d6/oasis/NSfracStep.py#L158-L159