Other sizes of fixed size vector

Gandalf supports fixed size vectors with sizes two, three and four. The functions described above for size three vectors are repeated for sizes two and four, both single and double precision, in the header files

      #include <gandalf/linalg/2vector.h>   /* double precision */
      #include <gandalf/linalg/2vectorf.h>  /* single precision */
      #include <gandalf/linalg/4vector.h>   /* double precision */
      #include <gandalf/linalg/4vectorf.h>  /* single precision */