era 0.2.0

First beta release, including:

• S3 vector class yr (era_yr) representing years with an era
  • yr() constructor
  • Functions for getting and setting the era of a yr object: yr_era(), yr_era()<- and yr_set_era()
  • Format and print methods for yrs
  • Vector arithmetic methods for yrs
• S3 record class era representing an era definition
  • era() constructor
  • Format and print methods for eras
  • Getter functions for era parameters: era_label(), era_name(), era_epoch(), era_unit(), era_scale(), era_direction()
  • eras() defining standard eras
• yr_transform() function for converting years between eras
• Function documentation and introductory vignette: vignette("era")
• A NEWS.md file to track changes to the package.