Quad16Shell Element Assumption

[bradenfrigoletto24]

Currently when using the Quad16Shell element an assumption is made that the nodes in the element utilize a non-uniform spacing. This assumption can be seen in TACSShellElementQuadBasis.h in TacsLagrangeLobattoShapeFunction(), where once the order is greater than 3, Gauss-Lobatto points are used for evaluating the Legrange basis. This assumption requires the underlying mesh nodes to use the same spacing. Currently no check or warning is issued to the user to notify them of this. If a uniform node spacing is used with the Quad16Shell, the Legrange basis will not be evaluated at the nodes properly and will lead to incorrect results. If the mesh nodes are spaced appropriately, the element should work as expected.