Add convenience input/output functions

[bryanwweber]

Output

• [x] ChemKED class instance to ChemKED YAML
• [ ] Arbitrary section of ChemKED class instance to YAML
• [ ] CSV to ChemKED datapoints record
• [x] ChemKED class to Pandas DataFrame (#16)
• [ ] ChemKED class to NumPy record or structured array
• [ ] ChemKED reference record to BibTeX
• [ ] CanSen input file

Input

• [ ] NumPy record or structured array to ChemKed class
• [ ] NumPy record or structured array to ChemKED datapoints record or ChemKED class instance?
• [ ] Pandas DataFrame to ChemKED datapoints record or ChemKED class instance?
• [ ] Excel file to ChemKED datapoints record or ChemKED class instance (via DataFrame?)?
• [ ] DOI to ChemKED reference record
• [ ] BibTeX to ChemKED reference record

[kyleniemeyer]

For the CSV, NumPy, Pandas, and Excel input options, we could have the user supply the reference DOI as well to have it automatically create the reference object

[bryanwweber]

Sure, we could probably develop that as a separate output function, then call it from any of these functions if the DOI is optionally passed