Added the class MinorSymLogLocator which is useful to locate minor ticks
on plots that have been scaled with the `symlog` scale.
I rediscovered this class, when I asked google for a solution to this
problem. It turned out someone on stackoverflow had the same problem and
a solution was posted. After trying to upvote, I found out that it was
actually me that posted both the question and the answer half a year
ago. Thank you stackoverflow for preserving my own thoughts!