add documentation and notes to AASite and RigidFragment

the notes spell out the difference between mass-weighted tensor of
gyration, which is used to compute physical properties like normal mode
frequencies, and the other tensor of gyration, which is used to compute
distances.
Merge branch 'fix_travis' into aametric_tensor
Merge pull request #93 from js850/aametric_tensor

add documentation and notes to AASiteType and RigidFragment