Write function to get energy levels.

I had written a function to obtain the energy levels from the line list
(fort.19 file).
Change name of functions.

The point here is to standardize the functions
Change what functions are imported.

Add some functions that are imported when one imports the package.
Add support to the get_energy_levels function

The script prints the energy levels, for one line query or all lines
query.
Explain how to check lines using energy levels.
Merge pull request #5 from gabraganca/ener_levels

Add support to obtain energy levels of transitions