The following functions are needed for each element type:

   mfs_chkelt       performs basic element checks
   mfs_defelt       defines the characteristic data of the element
   mfs_ke           computes the element stiffness matrix
   mfs_mce          computes the consistent mass matrix
   mfs_mle          computes the lumped mass matrix
   mfs_pltelt       plots the elements
   mfs_prtelt       prints the element results
   mfs_reselt       computes the element results

If some of these routines are not needed for some element type, dummy
routines have to be provided.
