C++ math function cleanup

C++ math function cleanup

math/functions.h now implements a number of old and new math
functions with either float, double, or integer arguments.
Manual SIMD versions of 1/sqrt have been tested with gcc and icc
on x86, Power8, Arm32 and Arm64, but with correct 'f' suffixes
on constants there is only 10-15% performance difference, so for
now we always use the system versions to avoid having this file
depend on config.h. Functions for third and sixth roots have
been introduced to replace many of our pow() calls, and the code
has been cleaned up to use the new functions.

Refs #1111.

Change-Id: I74340987fff68bc70d268f07dbddf63eb706db32