Add more readable name for isnan() from math.h [imported]

This is [Issue 812](http://code.google.com/p/arduino/issues/detail?id=812) moved from a Google Code project.
Added by 2012-02-10T18:03:34.000Z by [tom.i...@gmail.com](http://code.google.com/u/102867599453860860098/).
Please review that bug for more context and additional comments, but update this bug.

Original labels: Type-Enhancement, Priority-Medium, Component-Core
### Original description

There are some useful functions in math.h, especially isnan(). We should add a few defines to give them more beginner-readable names.