[Doc] Species doc attribute

[gabrielasd]

Currently the doc attribute of the Species class holds the documentation of the loaded dataset. It gets assigned by calling the function get_docstring

1- The docstrings are mere place holders for now, and its likely that the get_docstring function itself needs revision. Maybe instead of typing the information directly in the body of the function we should have it in a separate file(s)?

2- It could also be nice to add some functionality that appended a table with the actual properties available for the loaded specie.

The function get_docstring should grab the variable atomdb.{dataset}.DOCSTRING and return that. This allows us to keep all the information about each dataset in the same file, {dataset}/__init__.py.