Adds support to new way of inputting abundance to Synplot

The format of the values of chemical abundance that it is passed to Synplot can be sometimes enigmatic. This PR tries to solve this by introducing a new way of passing abundance values. The use can now uses a dictionary. For example, if the user want to use a value of Helium abundance of 10.93, normally it would write `'[2, 2, 10.93]'`. With the new way, the use can write `{'He':10.93}` or `{2:10.93}`. This is more explicit, specially the way with the chemical element symbol.

I also added a test function for this new class.