Test coverage

[joeroe]

• yr()
  • Returns class era_yr with valid input
  • Expect error for invalid era (needs validation function Add function for validating eras #7)
  • Expect error for multiple eras
• yr get and set functions
  • yr_era() returns correct era
  • yr_era()<- and yr_set_era() set correct era
• era()
  • Returns class era with valid input
  • Expect error for invalid input
• eras()
  • Returns correct row for exact match
  • Returns correct row for partial match
  • eras() returns all rows from era_table
  • All eras defined in eras() are valid
• era_parameters
  • Return correct output for all getters
• yr_transform()
  • dst_era is correct
  • All significant transformations return correct result
  • Expect error for incomparable units
  • No errors for otherwise possible transformations
• vctrs classes (suggestions: https://vctrs.r-lib.org/articles/s3-vector.html#testing-1)
  • expect_snapshot_output() of format.era_yr()
  • expect_snapshot_output() of format.era()
  • Method symmetry
  • Expect errors for inputs that can't be combined
• vctrs arithmetic
• era_year()

[joeroe]

851cc6d reached 98% (I'm not sure it's actually possible to reach stop_incompatible_op() in yr arithmetic methods), which I think is good enough.