maintenance and volume to biomass useablilty

Libcbm version 2.8.0

Changes since version 2.6.6

• minor code cleanup 0d41c3c
• docs, RStudio usage thanks @frandoodle
• add python-callable function to support running the low level CBM-CFS3 volume to biomass routine https://github.com/cat-cfs/libcbm_py/blob/main/libcbm/wrapper/volume_to_biomass/volume_to_biomass_wrapper.py
• linux build testing, now support ubuntu 22.04 and ubuntu 24.04. ubuntu 20.04 is no longer supported, but will likely still work, since github actions has discontinued this version
• support for smoother_enable flag in python workflows thanks @mfellows 9a91d80