Update standard atmosphere formula references

[dopplershift]

Just seems wrong to be citing Wallace and Hobbs for the U.S. Standard Atmosphere...

[jrleeman]

Well, there's this: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539.pdf

It's the 1976 update which may or may not have been what's in the book. Also, should we be using US or international standard? Close, but different.