RT#112758: set multiplot next Option

[mohawk2]

Andreas Pawlak writes on https://rt.cpan.org/Ticket/Display.html?id=112758:

Dear Maintainers,

Is it possible to add the

"set multiplot next"

option to PDL::Graphics::Gnuplot. This command is enabled since Gnuplot
4.7 and allow to skip a specific plot in the multiplot regime. Right
now, I can hand code this to my program using something like:

use PDL;
use PDL::Graphics::Gnuplot qw/terminfo gpwin/;

$win = gpwin('qt', persist => 1);
$x = sequence(101)/10;
$y = sin($x);

$win->multiplot(layout => [2,2]);
$win->plot($x, $y);
$win->plot($x, $y);
PDL::Graphics::Gnuplot::_printGnuplotPipe($win, "main", "set multiplot
next");
PDL::Graphics::Gnuplot::_checkpoint($win, "main");
$win->plot($x, $y);

# tested with Gnuplot 5.0.3

A subroutine "multiplot_next" would be great, somelike (not tested):

sub multiplot_next {
    my $this = _obj_or_global(\@_);

    unless($this->{options}->{multiplot}) {
        barf("multiplot_next: you can't, you're not in multiplot mode\n");
    }
       # standard syntax cheking
    _printGnuplotPipe($this, "main", "set multiplot next\n");
    $checkpointMessage = _checkpoint($this, "main");
   # standard checks
}

However, the version checking of Gnuplot should be included, otherwise
an error is thrown by Gnuplot.

Thanks and best regards,
Andreas Pawlak