Imposed lower limits to LAI and canopy height across ALL science

[JhanSrbinovsky]

@JhanSrbinovsky @ccarouge @bschroeter - this issue touches on a much bigger topic. Science-wise this routine needs to be linked into cable_roughness (including dust work in coupled), the connection between the lower limits on canopy height, LAI and roughness (so the various thresholds), #173 - and the general technical refactorisation.

Reconciliation across the repos is a good first step/aim - but does risk raising a lots of questions around what's the correct code and unintentional impacts of changes. What is in the code (all repos) currently is definitely incorrect - but which of the various options is least incorrect I can't say.

Originally posted by @har917 in #574