Writes class to handle Synplot abundance

This helps make the transition of abundance values from the Synplot
format to a dictionary format.
Adds support to more than one chemical element.

It is possible to retrieve more than one chemical element from a Synplot
abundance string.
Adds Periodic table.
Adds import to Synplot_abund class
Writes test fro Synplot_abund class
Returns abundance on Synplot format if set

If the input format of the abundance was already in Synplot format and
the user asked to put in Synplot format, it returns the original value.
Adds docstring to test function.
Supports the class Synplot_abund

The chemical abundance of the synthesis now accepts a dictionary
format.
Merge pull request #22 from gabraganca/synplot_abund

Adds support to new way of inputting abundance to Synplot