era 0.3.0

Second beta / CRAN pre-release

• Added new era definitions:
  • Islamic calendars (Lunar Hijri, Solar Hijri).
  • Julian calendar
• Reworked era definition scheme:
  • Unit is now represented by the era_year class, which describes its length in solar days as well as its name. Added functions for constructing and working with era_year objects: era_year(), is_era_year(), era_year_label(), era_year_days().
  • Direction is now coded as 1 ("forwards") or -1 ("backwards"). The previous character arguments still work but are deprecated.
  • Equality tests for eras now only check significant parameters (i.e. not "label" or "name"), allowing for coercion between functionally equivalent eras, e.g. yr(1, "BP") + yr(1, "cal BP") now works (with a warning) (#3).
• Improved yr_transform():
  • Can now convert between eras with different year units (#25).
  • Now has a precision argument, allowing the result to be rounded (#23).
• era and yr objects are now validated when constructed, using new functions validate_era()/is_valid_era() and validate_yr()/is_valid_yr() (#7 and #8).
• Minor changes/bug fixes:
  • Added this_year(), which returns the current year as a yr object.
  • era(NA) now returns an error, not a vector of all standard eras (#20).
  • All era arguments in functions can now accept a character vector (#20)
  • era(<era>) now returns an era with the same parameters (to enable the above)
  • Various additions to make the coercion hierarchy for era_yrs more consistent; most notably, the common prototype of era_yr, integer, and double, is now era_yr.
  • Fixed printing of NA eras (#9)
  • Errors now use the rlang format and are in general more informative (#26 and #28)