20140124 eth string helpers for measures 64247570

Adding a few helpers for measure-writing, especially:

- OpenStudio::toUnderscoreCase for using OSRunner::registerValue
- OpenStudio::toNeatString for printing floating-point numbers with a specific number of digits past the decimal (by default with comma separators)
- OpenStudio::toNeatStringBySigFigs for printing floating-point numbers with a specific number of significant digits

OpenStudio::numFractionalDigits may also be of interest to report-measure writers (for determining how many digits past the decimal they want to show in tables).