Octave figure window tuning for high DPI monitor

Octave figure window and its font look extremely small when being displayed in a high DPI monitor.

Even though the Qt graphics toolkit, which is used by Octave GUI, supports automatic scaling (this can be enabled by setting the Boolean environment variable QT_AUTO_SCREEN_SCALE_FACTOR to 1), it will only enlarge the size of the figure window and its widgets but not the font size.

Therefore, the visual effect of an Octave figure window should be manually tuned for the high DPI monitor.

Start Octave without Qt automatic screen scaling by creating a Bash alias:
```
alias octave="QT_AUTO_SCREEN_SCALE_FACTOR=0 octave --silent"
```

Increase the default font size and line width in ~/.octaverc.

set (0, "defaultaxesfontname", "Helvetica");
set (0, "defaultaxesfontsize", 24);
set (0, "defaulttextfontname", "Helvetica");
set (0, "defaulttextfontsize", 24);
set(0, "defaultlinelinewidth", 4);

Define a function for creating a new figure with a doubled size.

function CreateFig(h)
  if (exist("h", "var"))
    figure(h);
  else
    h = figure;
  endif
    
  current_pos = get(h, "position");
  ## Double the size of the figure window.
  current_pos(3:4) = current_pos(3:4) * 2;
  set(h, "position", current_pos);
endfunction

Now the enlarged figure window is shown below.