Implement principal stresses and strains

[acodcha]

Implement principal stresses and strains for the PhQ::Stress, PhQ::Strain, and PhQ::StrainRate quantities. These should have a Principal() method that returns the principal stresses/strains. Perhaps also FirstPrincipal(), SecondPrincipal(), and ThirdPrincipal() classes.

Implement these classes:

• PhQ::PrincipalStress: Contains a PhQ::Stress and a PhQ::Direction.
• PhQ::PrincipalStresses: Contains three PhQ::PrincipalStress objects. Has First(), Second(), and Third() methods.
• PhQ::PrincipalStrain
• PhQ::PrincipalStrains
• PhQ::PrincipalStrainRate
• PhQ::PrincipalStrainRates

Requires #213