Scripting prebuild notebooks

Pull request addresses several open issues including:

[x] Partially addresses issue #68 and #70 -- setting up GCS and CI/CD build workflow
- Adds the following scripts to aid in guiding the build process (in addition to a requirements doc for the build:
- build_phase1.py + data_to_download.txt
- build_phase2.py + data_preprocessing.py + ontology_cleaning.py
- build_phase3.py
[x] issue #72 -- conversion of the Data_Preprocessing.ipynb Jupyter Notebook into a script
[x] issue #77 -- creating script to map ChEBI to MeSH concepts
[x] issue #73 -- conversion of the Ontology_Cleaning.ipynb Jupyter Notebook into a script
[x] issue #43 -- adding command to GitHub Actions to publish Container to DockerHub for each release

callahantiff / PheKnowLator