All in a year: we should verify and clarify where the definitions of years used

[sefffal]

In some places we require years to be measured in Julian years (365.25) and in other places we use tropical years (365.244...). Decimal years are yet another kind of year used in a couple of places.

We should ensure these usages are all correct, and specify the convention used where appropriate.

[sefffal]

Side note: the HGCA is currently computed using decimal years. This should be adjusted to Julian years.

[sefffal]

All changes are done, now just need to document.