Improvements:

- Added Amber14SB and GAFF force fields.

- Updated physical constants (CODATA 2010 values).

- Some improvements in handling PDB files.

- Added phosphotyrosine residue definition.

Bug fixes:

- Force field parameters could be assigned incorrectly
  for dihedral angles whose central atoms have the same
  atom type.

- TIP3 now works with AMBER12.

- When replacing Cys by Cyx, the residue number remains the same.