dynamic age-class subset for plantation establishment depending on time step length

[flohump]

Please fill following information wherever applicable.

Purpose of this PR

Problem: Time steps with a length of more than 5 years cause gaps in the age-class structure of timber plantations.
So far, we manipulatd the level of v32_land in postsolve such that additions to plantations are distrubted over several age-classes (e.g. ac0 and ac5 for a 10 year time step). However, this is bad practice because it causes inconsistencies with carbon stocks, and problems with the carbonstock function in the magpie4 R library.
Now, there is a dynamic subset "ac_est", which depedends on the time step length (e.g. ac0 and ac5 for a 10 year time step). Together with the new constraint "q32_forestry_est" this allows to distribute the establishment over several age-classes within the optimization. This makes sure that carbon stocks (and all other variables) are consistent. Manipulations in postsolve are no longer needed. The carbonstock function in the magpie4 R library has been updated accordingly.
The counterpart of ac_est is ac_sub, which is also dynamic. This required a change in the declaration of some variables and paramters. Several unnecessary sets and parameters have been removed.

Performance loss/gain from current default behavior

No difference in run time.

Type of change

• [x ] Bug fix (Change which fixes an issue).
• New feature (Change which adds functionality).
• Minor change (Change which does not modify core model code i.e., in /modules).
• Major change (fix or feature that would change current model behavior).

How Has This Been Tested?

forestry_test start script.
One run before the bugfix. One run after the bugfix.

Default configuration additions/changes:

• Realizations:
• Scalars/Constants:
• Model interfaces added:
• Other changes:

Checklist:

• Self-review of my own code.
• [not needed for bugfix] Added changes to CHANGELOG.md
• [ x] No hard coded numbers and cluster/country/region names.
• [to my best knowledge: Yes ] The code doesn't contain declared but unused parameters or variables.
• [ x] In-code comments added including documentation comments.
• [ x] Compiled and checked resulting documentation with goxygen::goxygen() for the new/updated parts.
• Changes to magpie4 R library for post processing of model output (ideally backward compatible).

Special comments/warnings

• Notes

Resources_Land_Cover_Forest_Managed_Forest_Plantations.pdf
Emissions_CO2_Land_Land_use_Change-10.pdf

[abhimishr]

In principle it looks good to go but I added some requests.

Also can you edit the PR using the draft template from https://raw.githubusercontent.com/magpiemodel/magpie/7c481557d9ecf66582ab92c7e875259cad33228c/.github/pull_request_template.md

[abhimishr]

Thank you for addressing the small changes. For me looks good to go!

[abhimishr]

@tscheypidi we still need RSE side approval for bugfix PRs?