SEMODP ( pnum, ival, rval, ifail )
==================================

  Set modeller parameter

Receives:
  KI_cod_slmp              *pnum           --- parameter code
  int                      *ival           --- integer value of parameter
  double                   *rval           --- real value of parameter

Returns:
  KI_cod_error             *ifail          --- failure indicator

Specific errors:
  KI_bad_precision          Linear precision must be greater than zero
                            Angular precision does not lie within allowed range

Description:

  The value of 'pnum' is used to indicate which parameter to set and thereby
  which value ('ival' or 'rval') to use. At present this routine receives only
  real parameters.

  The valid values of 'pnum' and the corresponding parameters are:

  'pnum'    receive   parameter

  SLMPLP    'rval'    Absolute (linear) precision of modeller as a real value
                      greater than zero. Lengths and distances that differ by
                      no more than this value will be treated as equal.

  SLMPAP    'rval'    Angular precision of modeller as a real value greater
                      than zero. Angles, and values calculated from normalised
                      vectors, that differ by no more than this value will be
                      treated as equal.

  Modeller precision should not be changed from default values except where
  this is essential for compatibility with old transmit files.