Conversion between year units

[joeroe]

Adding non-Gregorian calendars complicates the definition and conversion of eras. We need to account for:

• The differing length of Gregorian vs. non-Gregorian years
• Differing lengths within an era – leap days etc. (?)
• Non-alignment of year starts, e.g. 1422 AH is either 2020 or 2021 CE, depending on where in the year we are

I haven't fully worked out how to do this. But some general principles:

• The scope of this package is years; we should avoid having to explicitly consider dates if at all possible
• In this context years are not dates. They are a measurement of the period of time elapsed from a particular epoch in a particular unit. They can be converted to dates, but only approximately.
• It follows from this that 2020 CE is actually "2020 Gregorian years from Year Zero", and that converting it to a non-Gregorian year will probably result in a decimal value.
• It's okay if conversions between eras are approximate (perhaps issue a warning about this though)
• It's important that conversions between eras are symmetric

[joeroe]

4f447ce added the core functionality. To do:

• Add tests
  • All possible transformations run without error
  • All possible transformations are symmetrical
  • Some specific values for key transformations
  • Invalid transformations throw an error
• Check & update documentation
  • Functions
  • Readme
  • Vignette

[joeroe]

Updated the documentation in de545c7.