User isotope is not drawn if it does not already exist in the mass table

[php1ic]

If the user gives the (N,Z) of an isotope to be marked as 'one of theirs' but it is not in the mass table, it will not be drawn.

An instance of Nuclide is created only for isotopes in the mass table. The function MassTable::readOWN just reads the given file, toggling Nuclide::own for the appropriate isotope if, and only if, it already exists.

Will need to either add a new instance to MassTable::theTable, or create a new container, for such cases.