Remove allocatable attribute from cvmix_ddiff

Only occurance was in wrapped computation of diffusivity coefficients (also
removed unnecessary "nlev" variable in background mixing)
Remove allocatable attribute from cvmix_shear
Remove allocatable attribute from cvmix_tidal
Remove allocatable attribute from cvmix_convection
Remove allocatable attribute from cvmix_math

This was used in the interpolation routine because we wanted to invert either a
3x3 or 4x4 matrix (depending on order of interpolation); I just forced the
matrix to be 4x4 but only the upper left 3x3 block is used for quadratic
interpolation.