code(METADATA Support): adding support for metadata for gaussian dataset

[Ansh-Sarkar]

@gabrielasd @msricher With respect to the discussions under #15 and #16 , with reference to #4 am opening a PR for the modifications I've made in the gaussian dataset for supporting the METADATA object.

Currently the changes have only been made to the gaussian dataset in order to facilitate feedback and reviews. The current PR consists of the following major changes:

• Bundling the PROPERTIES and DOCSTRING variables under the METADATA object, which can also be used in the future to extend more dataset related properties if required.
• Proper and well formatted command line based documentation.
• Proper and well formatted Sphinx documentation.

Preview of points 2 and 3 can be seen here.

Once the PR has been reviewed and all feedbacks have been addressed I can quickly modify all the other datasets to follow the same format and then move on to the second point related to property access.

[gabrielasd]

Hi @Ansh-Sarkar

Sorry for such a late feedback, the past months has been very hectic among other things because we were preparing for AtomDB's release.
@msricher and myself though your idea to facilitate documentation of the properties each database supported was a very good one. Unfortunately, as you know, AtomDB was not accepted by Google for funding. Nonetheless, if you would like, we wold be glad to continue having you to contribute to the project.