Gnuplot-based plotting for PDL

Changes for 2.025 - 2024-04-20

• Split version checking from `use Alien::Gnuplot` line for easier downstream packaging. Fixes #98.
• fixes for Windows (#89)
• use terminal "dumb" if probing shows "unknown" (#66)
• fix numeric-only strings for legend (#100) - thanks @d-lamb for report
• fix plot3d array-ref handling (#87) - thanks @djerius for report
• fix gplot array-ref handling (#86) - thanks @djerius for report
• add multiplot_next to skip one plot (#85)
• add documentation to UTF-8 encode text labels etc (#74) - thanks @Zaki for report
• add {multiplot,plot,multiplot_next,end_multi}_generate methods which return Gnuplot commands equivalent plot would execute (#95)