Add functions to load and calculate coessentiality

[abearab]

• [x] Upload current coessentiality data from https://github.com/kundajelab/coessentiality into CanDI dataverse
• [x] Draft load scripts to load coessentiality data
  • https://github.com/GilbertLabUCSF/CanDI/commit/4fd9dd077a80d8dfe98e9d8abc954f5c4a119f0e
• [x] Use CanDI data handler and define specified python class
• [ ] Include coessentiality calculation scripts based on kundajelab/coessentiality & broadinstitute/coessentiality