Validate sign of inputs to T Model functions.

[davidorme]

At the moment, there's nothing stopping people giving negative heights, dbh etc to the T Model functions. We probably should add this within the validation of the inputs to the functions of that module.

I'm not quite sure what level of user error we want to guard against though. Stuff that blows up the code, sure, but there is a runtime and code bloat cost to guarding against everything.