Add simple atom typers.

Add:
* Avogadro::Core::AtomTyper -- base interface for atom typing
* Avogadro::Core::SymbolAtomTyper -- produce element symbols
* Avogadro::Core::NameAtomTyper -- produce element names

Change-Id: I1744ba2c10246ea6ba54b361d336ea52bd14f138
Merge topic 'typer' into master

8aa884a6 Add simple atom typers.