A collection of timescale mappings.
Format
A named list of data.tables with 7 columns
- t
 time period as a ordered numeric ID
- year
 time year
- refyear
 reference year for the time period
- pred
 predecessor ID (t) of the time period
- tperiod
 time step of the time period, used for stochastic branch, equal to t when deterministic
- begyear
 first year of the time period
- endyear
 last year of the time period
