Remove dependency with periodic package.

SAGUI took the chemical symbols and atomic number from the periodic
package. Since I only have three chemical elements, I have put their
symbols and atomic number into a dictionary and removed the dependency
with the periodic package.
Remove information about PERIODIC package.

This package is not needed anymore. The user does not need to install
it.
Merge pull request #18 from gabraganca/periodic

Remove dependency with the Periodic package.