started word on yields for bonds
new additions

- renamed data/merchants_us_paper.csv to data/merchants_magazine_us_paper.csv
- renamed data/bankers_loans.csv to data/bankers_magazine_state_govt_loans.csv
- data/merchants_magazine_us_paper.csv

  - added price_gold
  - renamed value to price_paper

- data/bankers_magazine_govt_state_loans.csv

  - added price_gold
  - renamed value to price_paper

- added data/greenbacks.csv

  - added mean

- added data/greenbacks_fill.csv with missing values in greenbacks interpolated
reorganize files
working on adding yields
bump to version 2.0.0