Surface precipitation should not get negative

[vgro]

The current ve_run example data with the abiotic model on 1154-energy-balance-for-understory-vegetation produces this error:

ValueError: Surface precipitation should not be negative! Consider checking that the canopy water balance is correct.

Steps to reproduce the behavior:

1. checkout `1154-energy-balance-for-understory-vegetation'
2. install ve_example and change config to use [abiotic]
3. ve_run
4. See error

• OS: Sequoia
• Version 15.7.2

This is likely an issue with the canopy interception.

[vgro]

It seems to be caused by the leaf area index of the understorey which does not quite work with the interception. Excluding the understorey for leaf_area_index_sum in the interception resolves the error, but need to sense check that because the variable is also used in surface evaporation and we need to have a consistent assumption about the behaviour of the understorey, is it like a mini canopy or more like a moss.