Function to guess a default spatial unit. Also see activeSpatUnit() in
methods-instructions.R for a way to manually assign this default
Examples
g <- createGiottoObject()
#> python already initialized in this session
#> active environment : '/usr/bin/python3'
#> python version : 3.12
set_default_spat_unit(gobject = g, spat_unit = "cell")
#> [1] "cell"
