CCfits 2.4: new formula

This formula is for [`CCfits` 2.4](http://heasarc.gsfc.nasa.gov/fitsio/CCfits/), patched for OS X Mavericks as described in the [official documentation](http://heasarc.gsfc.nasa.gov/fitsio/CCfits/html/).
No. It's a slight rewrite of a function that had issues when compiling under clang. The patched version is functionally identical, except that it does not on a sometimes unimplemented feature of `std::valarray`.