use dry matter of different root diameter range to constrain the generated root

[junqi108]

To generate more realistic root architecture, we need to constrain the root by observed dry matter of different root diameter range besides the root length density (which is mainly fine root), see some nice values from the big dig trial.
We can convert the root length to dry matter using root tissue density or specific root length.
self.rtd: float = 0.05 if species == 0 else 0.15 # Root tissue density (g/cm3), different for grapevine

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