plcont: Contour plot, fixed linear mapping for Fortran 77

When called from Fortran 77, this routine has the same effect as when invoked from C. The interpretation of all parameters (see plcont) is also the same except there is no transformation function supplied as the last parameter. Instead, a 6-element array specifying coefficients to use in the transformation is supplied via the named common block plplot (see code). Since this approach is somewhat inflexible, the user is recommended to call either of plcon0, plcon1, or plcon2 instead.