Modified pal::Rainbow; added pal::RainbowSV

pal::Rainbow had hues 0-360, but both 0 and 360 are red, and this
led to color conflation between the smallest and largest values.
pal::Rainbow now uses hues 0-300, which is red through purple.

I also added pal::RainbowSV to make less-than-unity saturation and
value settings in a rainbow palette easier to create.
Band-aid fixed annoying vsearch warning
Added a few more docs for auto_twiddle
Added weird position handling to annotation position specs.

I encountered a situation in which a position of "-nan" was supplied
to an annotation and it croaked. This led me to provide the machinery
in this patch, which captures such weird values in specs and provides
sensible values for the drawing operations.
Many minor tweaks for postscript output
Another postscript tweak
Fixed a degenerate axis min/max edge case
Slightly better explanation of Prima::Application's import in ::ReadLine
Fixed twiddle() for overridden plot() in Simple.

Simple allows for the override of its plot() command. This enabled
App::Prima::REPL to very cleanly override how plotting behaves while still
fully supporting the Simple interface. Unfortunately, twiddle() was not made
aware of this possibility and caused App::Prima::REPL to hang in code that
called twiddle(). The new behavior returns immediately if plot() has been
overridden.
Fixed typo in Changes file
Fixed bug in PDL::Graphics::Prima::Palette::apply()
Filled in last gap in Changes file, finally
Title drawing no longer invoked for empty title.

It used to be that the title drawing operation was run if the titleSpace was
greater than zero, even if the title was blank or not defined. This leads to all
kinds of unnecessary behavior, and conceptually confusing behavior, which only
made it this far because the drawing operations were rendering empty strings,
and so no actually doing anything. This commit fixes the logic, and the block of
code for rendering the title is now skipped if the title is undefined or the
empty string.
Cleaned up a few return values; added function descriptions.

In an attempt to make the source more readable, I've added a few block comments
explaining the behavior of some of the code. I also added return values for some
of the functions which should, for consistency, return something.
Added titleFont capability; still need to debug and document
Fixed begin_paint issues and documented title font settings.
Fix recent flicker issues
Refactored plot drawing into draw_plot method.

The flicker in the animated plots was due to odd behavior with the
begin_paint/end_paint that was most easily solved by refactoring the
actual paint drawing functionality. This has the added advantage that
the draw-on-canvas method is now obvious to call and obvious to
override by a subclass.
Improved examples scripts; added a format-ticks example