Nodes of different colours represent the following:
Solid arrows point from a submodule to the (sub)module which it is
descended from. Dashed arrows point from a module or program unit to
modules which it uses.
Where possible, edges connecting nodes are
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large graphs.
reads carbon_layers.prt and sets cb_n_layers (number of soil layers
to include in per-layer carbon outputs).
carbon_layers.prt format:
line 1 - title (free text)
line 2 - header (free text, ignored)
line 3 - single integer: number of layers
the file is OPTIONAL. discovered via inquire; if missing, the routine
returns silently and cb_n_layers is left for output_landscape_init to
default to the largest soil layer count across all HRUs. no entry in
file.cio.