Add support for growth, removals, and mortality on changing land bases

[hunter-stanke]

rFIA v3.0.0 and older does not support estimation of growth, removals, and/or mortality (GRM) on lands that have reverted from non-forest to forest or been converted from forest to non-forest between plot visits. That is, GRM estimates only include plots that were forested at initial visit and remain forested at remeasurement. As EVALIDator suggests, this may give a more "realistic" view of the change components in forests that remain in the forested land basis. However, it is often totals that are of interest (e.g., total removals including those arising from conversion), and our current implementation lacks support for this.

I'll work on adding support to growMort and vitalRates this week. Likely in the form of an additional logical argument in each function - I'd rather keep totals as a simple dplyr::select and not allow it to affect estimation, consistent with all other functions. Open to alternative ideas and suggestions for argument names. I'll update this thread once all is up and running.