#47 symlog scaling

[Nico Schlömer]

matplotlib supports three scalings for axes:

• linear,
• log, and
• symlog.
  see http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.xscale.

linear and log are supported by pgfplots, symlog isnt'. Please add it.

(The idea for this is from https://github.com/nschloe/matplotlib2tikz/issues/38).

[Stefan Pinnow]

For the record:
Here is a question that also wants this feature on TeX.SX:

• http://tex.stackexchange.com/questions/348998
  (and marsupilam provided a solution which might serve as a good start for an implementation?)

And this I moved from the PGFPlotsToDo here:
http://stackoverflow.com/questions/3305865