replaced decimal module, xrchart_scale with autoformat, histogram x-axis ticks changed.

replaced decimal module, xrchart_scale with autoformat, histogram x-axis ticks changed.

During gtk3 osx debugging the decimal unit crashed. I replaced the decimal computations
with traditional tick interval computation plus a printf format string generation which is
adapted to the chosen tick style.

The xrchart scales writing are adapted to use this mechanism. If the tick text is too long
to fit in the interval, then the tick label uses the existing rotation by 45 degrees. I added
some space below the x-axis to make the tick labels visible. Maybe the tick label placement
could also be done in two rows for the even and odd ticks.

The histogram x-axis tick generation is changed from a tick per bin to the usual
normal x-axis tick generation. This allows easier visual bin border identification.
The bin width is adapted to the chosen x-axis tick scale. Any number of bins can now
be visualized. This is according to the spss examples I found for the histograms.