Logging in `BaseModel.from_config` methods.

**Describe the bug**

I've been going over the Variables PR #431 to try and push it along now that @vgro has broken the back of the update.

* I've updated all the variable descriptions
* I've updated all the model required vars sections.
* I'm going through the example log (which very helpfully prints out what data gets checked and set at each step - aren't we clever!) to validate all of the sections.

But as part of going through the log, I noticed a difference between the PlantsModel.from_config() and everything else. The other models do this (for example):

https://github.com/ImperialCollegeLondon/virtual_ecosystem/blob/a27518a4ba2f294ae3fd2de7607667205bc166bb/virtual_ecosystem/models/litter/litter_model.py#L171-L179

Plants does this:

https://github.com/ImperialCollegeLondon/virtual_ecosystem/blob/a27518a4ba2f294ae3fd2de7607667205bc166bb/virtual_ecosystem/models/plants/plants_model.py#L154-L169

Okay - so that is a little paranoid but there is a practical difference in the logging output. With plants, there is a message **after all the __init__ logging** indicating model success. The other models report an intermediate step - they only claim to have extracted the information needed - but the messages read a bit like the `__init__` has completed and then are followed by all the logging messages.

I think we should shift to using a Plants like setup for all model - it makes the logs clearer - but we should be consistent either way.

**To Reproduce**

Run `ve_example` and look at the log.

Run `ve_example` and inspect the log output, then compare `virtual_ecosystem/models/plants/plants_model.py` with `virtual_ecosystem/models/litter/litter_model.py` and the other model `from_config` methods. Make the model setup logging consistent so success is reported after initialization, and confirm the resulting log is clear and consistent.

Written by the indexing model from the issue text.