'Use gridplot' suggestion for PairPlot

[AllinCottrell]

Description of changes

• Use of gridplot means that all the graphics file formats supported by gretl can be used.
• It also means that usage via gretlcli in "display" mode works.
• A title option is added, drawing on relatively new functionality in gridplot.
• In case the user's specification calls for just a single plot, no "multiplot" is created.
• The graphical interface is enhanced, with more options.

[atecon]

Hi Allin,
thank you for your effort to contribute to the package. It took me a while to understand your large PR. This is why I've created another branch on which I implemented step-by-step your suggestions in order to understand things.

My new PR based on your suggestions can be found here:
#9

This new branch includes at least 99 % of your code changes. Only a few LOCs of existing code (mainly formatting) I did not change as you'll see.

I've added you as a co-author, too.

Once you've checked the other branch, we can upload the new package version.

I would suggest to close this PR #8 for now if you agree.