Improve assignment of PDB atom element from name

Not a common issue as x-ray files don't have hydrogens. Discovered this problem when parsing output from Rosetta - the element column is absent so the parser falls back to `PDB.Atom._assign_element()`. Previously, hydrogens with a leading digit were given element "".

Added a unit test to check that all types of hydrogens and other elements with H (i.e. Greek eta) are correctly assigned.